'A function' is also a very commonly used general term, i.e. outside of the context of mathematics. It describes how something is used, behaves, or the role it serves in some kind of described system. For native speakers, we learn and use 'function' in this general sense far earlier than we do when in the context of mathematics. 'In this job, I function as a go-between for the sales and accounting departments.' 'Primary functions of a friendship are moral, psychological, and social support.' 'In that situation, you seem to function mostly as a babysitter.' 'The function of the throttle of the riding lawnmower is to allow the operator to control the speed of the mower.'