Third person singular present/future indicative of gwneud. A bit strange to have it as a word on its own, though - it could be translated in several ways depending on the context.
In the colloquial language, only as an answer - 'Yes, (she/he/it will)' Otherwise, it should be accompanied by its subject:
- Gwnaiff hi orffen y gwaith heno - She will finish the work this evening
- Gwnaiff Siân y gwaith 'na - Siân will do that work
- Wnaiff hi orffen hynny heno? Gwnaiff. - Will she finish that this evening? Yes.
Things are less clear-cut in the formal language, but that often uses different patterns anyway.