Freewriting entails jotting down whatever comes to mind in an attempt to get creative juices flowing. It is meant to be private writing (seen by no one other than the writer) but may be shared. Freewriting may be focused or unfocused.
Unfocused freewriting requires every thought to be transferred to the writing medium. For instance if the thought of aardvarks wielding umbrellas feasting on giant roly-polys while riding the rails enters your mind write about that until your thoughts change to exploding pogo-sticks (or whatever else happens along in that head of yours). In short just go crazy with your writing and see where it takes you.
Focused freewriting is very much like unfocused freewriting except a topic is chosen prior to writing. This topic may be whatever and as broad as one chooses. It is important to note that what is written need not strictly be about the topic. For example if one chooses their favorite band as the topic and they begin to think about the effects of various materials on the degradation of sound waves, they might add that to what has already been written. However, if one goes from their favorite band to the proper methods to employ when checking squash for freshness, information about the squash need not be included (unless the band did a song about checking squashes for freshness). The key is to maintain relevance to the topic.