Prestiging gives you awesome rewards, like a super powerful damage boost for all Superstars and perk points which you can spend on upgrading your Superstars, along with access to Live Events. In exchange for these amazing rewards, you must reset your progress back to Stage 1.
Even though you’re back at the start, you still keep all your cards, gold, attire, spent perk points, achievements and objectives. The only things you lose are your levels, cash, and passive skills, which you’ll earn again while playing through the game. The road back to Stage 100 will be much easier this time around, too.