Multi-tenant simply means shared with someone else, like having a roommate in an apartment. In the infrastructure context, it refers to one physical server that has software from multiple different people (or companies) running on it. Which could be hundreds of roommates depending on the size of the server.
Multi-tenancy is central to why cloud was such a revolution in the 2010s: it let providers like AWS rent space on a server without having to allocate the entire server to one person. Unless you pay extra, most of the cloud services you use are probably sharing hardware with someone else.