"What will you do tomorrow?"
It's the conjunctive stem (called 'renyoukei' in Japanese) of the verb "to do". You use this stem to attach all kinds of suffixes that change the verb's purpose or tense, such as ～ます (polite speech), ～たい ("want to ~"), ～ない (negation), ～よう (pseudo-future).
In plain/informal speech (a.k.a. the 'dictionary form' or 'rentaikei') "to do" = する, but this can't be conjugated to anything else.