Meaning of ? mark in SQLite

It's a query parameter. The value isn't known at compile time - it is determined at runtime by using the contents of the variable $key.

Other notations also exist for specifying parameters:

 ?NNN    A question mark followed by a number NNN holds a spot for the NNN-th         parameter. NNN must be between 1 and SQLITE_MAX_VARIABLE_NUMBER.    ?    A question mark that is not followed by a number holds a spot for the         next unused parameter.:AAAA    A colon followed by an identifier name holds a spot for a named         parameter with the name AAAA. Named parameters are also numbered. The         number assigned is the next unused number. To avoid confusion, it is         best to avoid mixing named and numbered parameters.@AAAA    An "at" sign works exactly like a colon.$AAAA    A dollar-sign followed by an identifier name also holds a spot for a         named parameter with the name AAAA. The identifier name in this case         can include one or more occurances of "::" and a suffix enclosed in         "(...)" containing any text at all. This syntax is the form of a         variable name in the Tcl programming language. The presence of this         syntax results from the fact that SQLite is really a Tcl extension         that has escaped into the wild.

That's a parameterized query. The ? is a placeholder for the actual value stored in $key.

Typically used in what's called. stored procedures or prepared-statements