A True Story.
A client recently called to say she could not print, get her email, or just use her iMac since it was too slow. When I arrived, there was just 113MB of free hard drive space.
Your Mac should have at least 10 to 20 percent free hard drive space, just to do it's job. You can see how much free space is available at the bottom of any Finder window. If you're running low, it might be time to delete unnecessary files, move files to an external hard drive, or replace the internal hard drive with another that has a larger capacity. If you're moving or deleting files, make sure they're yours, and NOT system or program files. Running low of hard drive space could also be due to system-level problems, such as log files getting too large or numerous.