A disk quota is a limit, set by a server administrator, which prevents applications on your server to occupy more disk space than you allow them to. The function of using disk quotas is to allocate limited disk space in a reasonable way so you know that your project is not using more space than it should.
You can enable the disk quota from the VPS Management section. To configure exact values, open a control panel that supports this feature such as cPanel or Plesk and specify your desired values. This feature is really useful if you have quite a few users and you want to limit their account storage space.
NOTES:
Keep in mind that hitting disk quota can make your VPS inaccessible so proceed with caution.