A hold is like a block or restriction that is placed on your account due to missing information or documentation, or not complying. If you have not declared a major, if you have not paid tuition, if you have not submitted paperwork, there are numerous things that can result in a hold on your account, these are just a few of them.
Depending on the severity of the academic hold you, may be prevented from doing things such as registering for classes. Once the issue that has caused the hold is resolved, an administrator will be the one to remove it from your account.