I have to admit that I don't use free writing as often as I should, and I'm sure that my writing suffers as a consequence.
What's free writing?
It's a pre-writing warm-up discovery exercise. It takes your writing in new directions.
You begin with a phrase, or a word. For example, your phrase could be: "This summer, I ..." then you keep writing for five minutes.
When you free write, it's important not to think about what you're writing. Just write the first words that come to mind, and keep writing. Don't cross out typos, don't correct your grammar, just keep writing. If no words spring to mind, you can whine on paper: "This is a stupid exercise, I don't know what to write, I ..." etc.
You can free write on your computer, or by hand.
I usually set a timer, and write on a yellow legal pad with a fountain pen.
At the end of five minutes (you can opt to free write for a shorter time, say two minutes, or a longer time, say ten minutes) you can either discard your exercise, or use it in various ways.
Using your free writes
I often look at what I've written, circle five words, and then cluster on those words.
Be creative in using your free writes.