macOS needs free space to work properly, and a nearly full drive causes slowdowns and odd behaviour well before it actually runs out.
- Open the Apple menu, then System Settings, then General, then Storage to see what is using space.
- Empty the Trash, which does not free anything until you do.
- Look at the Applications and Documents categories for large items you no longer need.
- Check the Downloads folder — it accumulates far more than most people expect.
- If you use Photos or iCloud Drive, check whether originals are stored locally.
Be cautious with third-party "cleaner" apps. Some remove files that macOS needs. If you are unsure what something is, leave it and ask us.
This is general guidance, not a guaranteed fix. The same symptom can have different causes on different machines. If you are unsure about a step, or it does not help, raise a ticket and tell us what you tried.