Final Cut Pro X inaccurately tells me I am out of disk space?

This message telling me I am "Out of Disk Space" and that "The Disk 'Macintosh HD' is almost full. Please clear some space before continuing." Keeps popping up and it wont let me work. But I have a lot of free space left on my computer. Why is this happening? How can I fix this? And can you explain in simple terms because I'm new to this. Thanks so much.