To do work. As far as I am aware it's more for chores and things. Chinese verbs are often made up of a verb and a noun. 吃飯 - this literally means "to eat food", but it just means "to eat". 幹活 is one of these.

work in the daily life , when we are going to work ,we often say:"我要去干活儿了."

It means to work.