What does “作” mean in this context?
“作”, when as a verb in this context, means "to make trouble".

“作”, when as an adjective in this context, means "loving to make ridiculous troubles (being likely to be a trouble-maker)".

“大作” means to make big troubles, usually causing bad consequences.

“小作” means to make small troubles, normally not really harmful.

"她是个很作的女孩” means she is a trouble-making ridiculous girl.(For example: She wants gifts every month and will be very angry if not; she gets upset ridiculously when you contact some other young girls, etc..)

By the way, with this meaning, officially, it should be pronounced as "zuò" even though most normal native speakers pronounce it as "zuō".