You can tell Florence which days and times you're usually free to work. You'll find this in your profile settings in the app.
Setting your availability doesn't book you onto anything, and it doesn't commit you to working. You still choose every shift yourself. It simply helps us match you with work that fits your life, so you see more of what's useful and less of what isn't.
It's worth keeping it up to date if your circumstances change, for example, if you go back to studying, or your childcare changes, or you pick up a permanent job elsewhere.
You can still apply for any shift you can see, even if it falls outside the availability you've set.