This message indicates that the file you are opening is on a drive or disk that is close to being completely full. This is potentially dangerous, as it will eventually lead to data corruption.
For most users, this message results from the file being opened from a floppy disk. Floppy disks only hold 1.44 MB (MegaBytes) worth of data, which is not much compared to the average size of our program's data files. Clicking "OK" to this message will typically result in the file opening properly into your database program.
Rather than continuing to use the file from the floppy disk, you should use your computer's hard drive or a server location to save your files.