A Static Ability is one that, once it is employed or put into effect, simply is always in effect until the endgame. This ability is opposite of Activated Abilities.
Static Abilities include some Evergreen Keywords, to name a few:
However, these keywords are not always static. If you must pay extra mana for them, they are considered Activated Abilities. In example:
Sabretooth Tiger has First Strike as a static ability. Even though Mantis Engine can have both First Strike and Flying as abilities, you have to pay for them; therefore they are Activated Abilities.