Free writing is a technique to help a writer get started on a piece they need to produce -- a story, report, article, letter or whatever -- when they are having difficulty, or to assist in overcoming writer's block.
The idea is to sit and write, without stopping, for 10 solid minutes. It doesn't matter what you write -- stream of consciousness, banality or pure gibberish, the intention is to concentrate on the process of writing and the act of getting words down on paper.
The purpose of doing this is to get the writer into the correct frame of mind to write, and allow inspiration to strike, not to produce anything meaningful -- although, often, as a side benefit, a piece results that can be either used, as it stands, incorporated into whatever you were planning to write (the ideas often start to come during this period of free writing) or put aside and used as the basis for a future story, article or whatever.
Generally, it works, although it may, occasionally, be necessary to repeat the 10 minute exercise.