A "function" is a sequence of programming instructions that makes a calculation based on data you provide. Functions can answer questions, such as how many days have passed between two dates, or the distance between two cities.
We are currently primarily focused on functions related to Wikidata Lexemes. The Lexicographical data from Wikidata and functions to process it are essential for the goal of an Abstract Wikipedia. A tutorial on how Wikifunctions can be used with Wikidata right now is available.