First strike is an ability introduced in old sets. It is an evergreen ability, meaning that it can be in any set.
Ability: This creature deals combat damage before creatures without first strike.
Despite looking confusing at first, this ability is quite simple. Normally, creatures without first strike deal combat damage during the combat damage step. However, creatures with first strike deal their damage in a step right before the normal combat damage step, called the first strike damage step. That means that a 3/2 creature with first strike will kill a 3/3 creature without first strike, but will not be killed.