The wrapper, binder, and filler refer to the individual leaves that make up the cigar you're enjoying.
The filler leaves of the cigar are the inner-most leaves and make up a majority of the tobacco inside. Filler leaves are bunched together by buncheros at the factory.
These filler leaves are held together with a binder leaf - a whole leaf encasing all of the filler tobacco and holding them tightly together.
Finally, and perhaps most importantly, the wrapper. This single leaf is the outermost cover of the cigar, and must be high-quality in both performance and aesthetics.