Reinvigoration is a Fatigue recovery modifier. Units will recover 1 point of Fatigue at the start of each of their turns, 5 points if they are unconscious (from having 100 points of Fatigue or more). The amount of Reinvigoration a unit has directly increases how much Fatigue they will recover on each of their turns.
You can apply Reinvigoration to your Blessed units for 2 Bless Points per point of Reinvigoration. If you don't want to do that but still want early Reinvigoration, your early sources of Reinvigoration are:
Mid-to-late-game you will get other options. Does anyone wanna list them?
The early-game 1 spell does not provide Reinvigoration; it fully recovers the caster's Fatigue instead, for the low price of one Blood Slave. The late-game 5 spell does not provide Reinvigoration either, instead reducing the Fatigue accrued by the caster and their friends for one battle for a single Nature Gem.