It is a relative form of 'mae'. You may also see it in its more formal form of sydd
- Mae e'n hoffi hufen iâ - He likes ice-cream
Pwy sy'n hoffi hufen iâ? - Who (is it that) likes ice-cream?
Mae e'n mynd allan heno
Pwy sy'n mynd allan heno? - Who (is it that) is going out tonight?
Dyna'r dyn sy'n gyrru drwy'r pentre'n rhy gyflym - there is the man who drives through the village too fast
- Hi sy'n casáu afalu - She (it is who) hates apples