Disk space is the amount of data allocated the a specific client. For instance, if a client is allocated 2GB (2,000MB) of disk space, then he or she could store up to 2,000 1MB pictures on that server.
When calculating disk space usage, it is usually the combination of all the files saved on the server, plus databases and email space.
If a client exceeds their disk space limit, the server will not allow them to upload any additional files.