Resilient is a passive feature provided by the Feat of the same name. Pick an Ability score and increase it by 1. You gain your Proficiency Bonus to your Saving Throw of the same ability.
Grants one of:
Description
You increase an Ability by 1, to a maximum of 20.
Gain Proficiency in that ability's 
Saving throw.
How to learn
Granted by the feats: Resilient (Feat)