Barkskin

2nd-Level Transmutation Spell (Druid, Ranger)

Casting Time: Bonus Action
Range: Touch
Components: V, S, M (a handful of oak bark)
Duration: 1 hour

You touch a willing creature. Until the spell ends, the target's skin assumes a bark-like appearance, and the target has an

An Armor Class (AC) is the target number for an attack roll. AC represents how difficult it is to hit a target.

Your base AC calculation is 10 plus your Dexterity modifier. If a rule gives you another base AC calculation, you choose which calculation to use; you can't use more than one. See also Attack Roll.

of 17 if its AC is lower than that.

Source: Player's Handbook, p. 245