Refers to the process or state of recovering from illness or injury. Often used in contexts like 治りが早い (quick recovery) or 治りが遅い (slow recovery).
My cold is slow to heal, and I still feel sluggish.
Young people recover from wounds quickly.
Noun form of the verb 治る (なおる), derived from the 連用形 (continuative form) of the verb. The verb itself is of native Japanese origin.