What's the meaning of multiple const qualifiers?

In your example all but the top level of indirection all const qualified.

const char            /* const qualified base level */    *const            /* const qualified level 1 */    *const            /* const qualified level 2 */    *                 /* regular level 3 */    foo(int bar);

foo is a function that accepts an int argument and returns a regular pointer.
The pointer it returns points to a const qualified pointer
which, in turn, points to another const qualified pointer
which points to const qualified char

If you have a multilevel pointer, you have several pointer variables. Example:

char*** foo;

is accessed like this:

| foo | pointer1 | pointer2 | string |   |    ^     |    ^     |    ^    \___/      \___/      \___/

You can qualify each of the four locations in memory as being const, as in the declaration

const char *const *const *const foo;

However, best avoid becoming a three star programmer.

Hah - the confusion around const.

Basically, if it qualifies a pointer, then it applies to the pointer immediately to its left. For example:

char *const c;

makes c a read-only pointer to char, whereas

const char *c;

Makes c a pointer to a read-only char, though you can change where c points to.

Of course, both the pointer and what it points to can be made read-only:

const char *const c;

This makes c a read-only pointer to a read-only char. With the former declaration, you can modify *c, but you can't modify c (i.e. you can't make it point to somewhere else). With the second, you can assign to c but not to *c, and with the third, you can't modify where it points to, nor can you modify the pointed-to object.

This extends to multiple levels of indirection. For your case, you might want to regroup the const keywords with the matching pointer that they are qualifying. This is mainly a whitespaces issue, but it would be like this:

const char *const *const *foo(int bar);

Which makes foo return a pointer to a read-only pointer to a read-only pointer to a read-only char. Read that carefully: note that given this piece of code:

const char *const *const *ptr = foo(0);

Then you know that assigning to ptr is legal, but any other assignment is illegal, namely, you can't assign to *ptr, **ptr, and ***ptr.