The pre-processional is when the groom walks the aisle with - usually - his mother, but can also include best man, groomsmen, maid of honour, bridesmaids, and flower children.
It's not mandatory to have a pre-processional, and in fact, most wedding couples prefer not to have one.
In that case, the groom will stand at the front, waiting for his bride to arrive.
If you do have a pre-processional, and you didn't contract life musicians, then you need to upload the music for the pre-processional in MP3 format.
Related pages on our site or useful links to other sites :Music for the wedding ceremony (page 16-28)