It's a lab, in your home! Specifically a technology lab, usually consisting of a server (or servers) running different applications and/or systems for either functional home purposes or recreational purposes (or both!). A homelab could be a small raspberry pi running one simple script or it can be a rack (or racks) of enterprise servers running more things than one could possibly imagine, just as long as it's for personal usage and in your home, it can be a homelab.