It looks like the grammar indeed accepts all words of the form $[b+a]^nca^n$ (which means, all words that start with any sequence of $n$ $b$'s and $a$'s, and then a single $c$ and afterward exactly $n$ times the letter $a$).
To show why to try to show the two following things:
every word accepted by the grammar must be in such form
every word with such form has a derivation sequence in the grammar.
The first statement can be easily proved by induction (over sequence derivation length) if you notice that each derivation of $S$ adds only one element to both sides.
The second statement can be much more easily proved, try to think of what derivations are necessary to create such words.