In fighting games, proration is a numerical value given to a certain attack that shows what percentage 'damage penalty' you get for using the attack in a combo.
For example, let's say you have attack A with 100% proration and attack B with 80% proration, and both can combo into each other interchangeably. Chaining A->B will have no effect on the damage output by attack B because attack A has 100% proration. However, chaining B->A will have an effect on the damage output by A; B has 80% proration, so attack A will only do 80% damage in this combo. Proration effects can also be stacked, which is the reason why button-mashing comboers often find themselves doing very little damage near the end of their string of attacks.
To find out what moves are good to start combos with, you should not only know how slow the move is and what it chains into, but you should also keep in mind the proration it has.