Translation:She works in the supermarket because she needs money.
Airgid is the genitive of airgead. You will see it used in the phrase a chuid airgid, meaning "his (share/portion of) money" or bonn airgid meaning "silver coin" (or "coin (made) of silver"). Bonn can also mean "medal".
The genitive is also used after the verbal noun, so ag tuilleamh airgid is "earning money".