Reading wikipedia it seems straightforward to me - it is a function that, when convolved with something, makes it smoother in an appropriate sense. The use it gives seems eminently easy to understand too - convolving step functions (indicator functions) one obtains 'nice' functions constant on given compact sets, and zero outside an arbitrarly small open set around the compact set. Personally, I don't see any use for it, but it isn't mathematically complicated.
Perhaps, since you find that wiki article incomprehensible, you might go through it and ask for elucidation on the precise parts that you don't get.