What is a computer science conference, really? I understand that in the field of computer science, publishing in a conference is the default, as opposed to other fields where it's normal to publish your research in a journal. But does the word conference here carry the same meaning as it does in other fields? 

If your paper is accepted in a conference in computer science, does it simply mean that it will be published in a proceedings rather than a journal? Or does it really mean that you have to travel to a physical conference and present your work as a talk? If the latter, what happens if you have multiple papers accepted at different conferences in the same year but can't afford to travel to all of them? I imagine that in this situation, trying to publish any work would result in a scheduling nightmare, both on the part of conference attendees and organisers. If this is the case, how did this situation arise and why is it allowed to continue? 

Or have I got the wrong end of the stick and in computer science, the word conference is just a synonym for journal?
Most computer science conferences expect that, except in extraordinary circumstances, at least one author will present the paper in person. The paper is also, in most cases, published in the proceedings, possibly on paper and distributed to some set of people, such as a special interest group. 

The reason for this is that we highly value collaboration and face to face conferences give us a chance to sit and talk as well as interact with authors. 

If you publish in a lot of conferences you need to plan for the travel. Grant funding is one source, as is university funding. I think that personally funding your travel is not especially common, but possible. 

Note, also that many conferences attendees work in industry, not academia, where they do research of various kinds. It is common to see folks from IBM Research, or Microsoft Research, for example at many ACM conferences. They are funded by their companies. 

One option for a prolific publisher (we should all be so lucky) is to do a lot of collaborative work so that if you can't attend, say, SIGPLAN POPL 2021, then one of your coauthors will be able to. 

But no, it isn't just another way of saying "journal". 

I should also note that quite a few people attend such conferences even when they have no part in the program. They are just there for the collaborative opportunities and for informal meetings.