aiff is the present/future form of the verb mynd (to go), meaning 'he/she is going/goes' or 'he/she will go'.

Mynd is an irregular verb in Welsh, like gwneud (to do), cael (to have) and dod (to come).

Here are the present/future tense forms of the four irregular verbs:

Mynd	Gwneud	Cael	Dod
Af i	Gwnaf i	Caf i	Dof i
Ei di	Gwnei di	Cei di	Doi di
Aiff e/o	Gwnaiff e/o	Caiff e/o	Daw e/o
Aiff hi	Gwnaiff hi	Caiff hi	Daw hi
Awn ni	Gwnawn ni	Cawn ni	Down ni
Ewch chi	Gwnewch chi	Cewch chi	Dewch chi
Ân nhw	Gwnân nhw	Cân nhw	Dôn nhw