。
Let's take a little break under the tree.
Literal
Tree [possessive-の] under [at-で] a-little rest-[let's-よう] [emphatic-よ].
休もう is the volitional form of 休む ('rest') — plain 'let's X' or 'I'll X.' Final よ adds a friendly nudge, softening the invitation. ちょっと ('a little') is one of the most common softeners in casual Japanese, dialling back the commitment of any verb it modifies.