。
It's gotten cool.
Literal
Cool became-[polite-ました].
A basic state-change sentence. 涼しい ('cool' — for pleasant coolness, usually autumn weather) + くなる (state change of い-adjective). Distinguish from 寒い ('cold,' as a temperature unpleasant to the body) and 冷たい ('cold' to the touch, e.g. cold water).