A hotfix is an urgent, targeted fix for a serious problem, often released outside the normal release process. It changes as little as possible, so that it can be made and tested quickly.
The term comes from fixing something while a system is live. Vendors issue hotfixes for problems such as an actively attacked vulnerability, a bug that breaks a key feature or a failure affecting many customers.
Because it is made in a hurry and often for a specific version, a hotfix may not get as much testing as a regular release. It is usually folded into the next regular release, which then includes the fix along with other changes.
If you install a hotfix, note which version it applies to and what it changes. When the next regular release comes out, install that one and confirm it contains the fix. Hotfix is used loosely, and some vendors use it for any small fix, so check what the notes actually say.