If your computer is displaying the error message that the hard drive is running out of space, or "Your startup disk is almost full." You need to make more space available right away.
If you don't strange things are likely to happen - like applications not working properly, perhaps even quitting unexpectedly, and the computer tends to get very slow.
Good news is - its easy to free up space... lets start with the basics:
Empty your Recycle bin
Delete files you don't need for example in the Downloads folder
- Reference: https://support.apple.com/kb/PH13806?locale=en_US
- Search Finder for large files and delete them if they aren't needed
- Reference: https://www.youtube.com/watch?v=RVvRPey5Ht8