Yes it is the drive belt
The usual reason for this is the belt jumps off the rear pulley and some on puts it on backwards .
But before we start a guessing game and every one chimes in with their favourite fix for a problem you don't have, we need the full model number from the tag under the seat.
Your mower could have any one of a dozen different drive systems and it helps if we know exactly what you have before we try to diagnose it .