How to understand the equal sign '=' symbol in IMAP email text?

In a nutshell, an equal sign at the end of a line indicates a soft line break. An equal sign followed by two hexadecimal characters (0-9, A-F) encodes a single octet (byte).

This encoding scheme is called "quoted printable" and is defined in section 6.7 of RFC 2045. See items (1) and (5), in particular.

6.7.  Quoted-Printable Content-Transfer-Encoding   The Quoted-Printable encoding is intended to represent data that   largely consists of octets that correspond to printable characters in   the US-ASCII character set.  It encodes the data in such a way that   the resulting octets are unlikely to be modified by mail transport.   If the data being encoded are mostly US-ASCII text, the encoded form   of the data remains largely recognizable by humans.  A body which is   entirely US-ASCII may also be encoded in Quoted-Printable to ensure   the integrity of the data should the message pass through a   character-translating, and/or line-wrapping gateway.   In this encoding, octets are to be represented as determined by the   following rules:    (1)   (General 8bit representation) Any octet, except a CR or          LF that is part of a CRLF line break of the canonical          (standard) form of the data being encoded, may be          represented by an "=" followed by a two digit          hexadecimal representation of the octet's value.  The          digits of the hexadecimal alphabet, for this purpose,          are "0123456789ABCDEF".  Uppercase letters must be          used; lowercase letters are not allowed.  Thus, for          example, the decimal value 12 (US-ASCII form feed) can          be represented by "=0C", and the decimal value 61 (US-          ASCII EQUAL SIGN) can be represented by "=3D".  This          rule must be followed except when the following rules          allow an alternative encoding.    (2)   (Literal representation) Octets with decimal values of          33 through 60 inclusive, and 62 through 126, inclusive,          MAY be represented as the US-ASCII characters which          correspond to those octets (EXCLAMATION POINT through          LESS THAN, and GREATER THAN through TILDE,          respectively).    (3)   (White Space) Octets with values of 9 and 32 MAY be          represented as US-ASCII TAB (HT) and SPACE characters,          respectively, but MUST NOT be so represented at the end          of an encoded line.  Any TAB (HT) or SPACE characters          on an encoded line MUST thus be followed on that line          by a printable character.  In particular, an "=" at the          end of an encoded line, indicating a soft line break          (see rule #5) may follow one or more TAB (HT) or SPACE          characters.  It follows that an octet with decimal          value 9 or 32 appearing at the end of an encoded line          must be represented according to Rule #1.  This rule is          necessary because some MTAs (Message Transport Agents,          programs which transport messages from one user to          another, or perform a portion of such transfers) are          known to pad lines of text with SPACEs, and others are          known to remove "white space" characters from the end          of a line.  Therefore, when decoding a Quoted-Printable          body, any trailing white space on a line must be          deleted, as it will necessarily have been added by          intermediate transport agents.    (4)   (Line Breaks) A line break in a text body, represented          as a CRLF sequence in the text canonical form, must be          represented by a (RFC 822) line break, which is also a          CRLF sequence, in the Quoted-Printable encoding.  Since          the canonical representation of media types other than          text do not generally include the representation of          line breaks as CRLF sequences, no hard line breaks          (i.e. line breaks that are intended to be meaningful          and to be displayed to the user) can occur in the          quoted-printable encoding of such types.  Sequences          like "=0D", "=0A", "=0A=0D" and "=0D=0A" will routinely          appear in non-text data represented in quoted-          printable, of course.          Note that many implementations may elect to encode the          local representation of various content types directly          rather than converting to canonical form first,          encoding, and then converting back to local          representation.  In particular, this may apply to plain          text material on systems that use newline conventions          other than a CRLF terminator sequence.  Such an          implementation optimization is permissible, but only          when the combined canonicalization-encoding step is          equivalent to performing the three steps separately.    (5)   (Soft Line Breaks) The Quoted-Printable encoding          REQUIRES that encoded lines be no more than 76          characters long.  If longer lines are to be encoded          with the Quoted-Printable encoding, "soft" line breaks          must be used.  An equal sign as the last character on a          encoded line indicates such a non-significant ("soft")          line break in the encoded text.