Its used to output to an external monitor, and sometimes includes sound as well as video depending on your hardware. Its the cable type ksiemb mentioned, which you would need to connect it to anything.
If you don't know what it's for, you don't need to use it for anything yet. It needs another device that has an HDMI interface which you would know if you had one that had that, and therefore you would not be asking this question. So logically, since you want to know what it is, you have no use for it.