In Football, a Finisher is an attacking player who is good at scoring.
In FIFA Ultimate Team, Finisher (Abbreviated as FIN) is a consumable chemistry style card which is focused on attacking and adds maximum two values (+2) to a player card's SHO and PHY stats on 100 Team Chemistry and 10 Player Chemistry.
When a player card has "FIN" written on it, it means a Finisher Chemistry Style card is applied to that player.
Below is the full stats boost details for Finisher chemistry style card on 100 Team Chemistry and 10 Player Chemistry in FIFA 17:
+5 Finishing, +15 Shot Power, +15 Long Shots, +10 Volleys, +10 Penalties, +15 Jumping, +10 Strength, +10 Aggression