A very broad word, admitted! Documentation can be anything writen or recorded as sound or video, it can be electronic or in a physical shape, and it can be about any topic you can think of. But... it must somehow describe the topic.
So, documentation is a description. Maybe describing how to use a product, maybe trying to tell the essence of an idea, or maybe being some kind of historical record - which could have the shape of a log from a computer system or a report displaying the current or planned settings of same system.
A set of documents
In daily life we often speak about documentation as, indeed, documents - a few years ago typically on paper, now more often as electronic files. And the systems documentation of a computer system then comprises of a bunch of different documents, such as a systems overview, the installation guide, a user's manual, a systems administrator's manual and a reference manual.