Hyper Log Log — link to wikipedia

I heard this is an important algorithm to learn to get to 1000 rating, Could anyone break it down in simple terms, or maybe share an analogy? Like, something a 900 rated would understand?

Q. What exactly is it doing? I mean, how does it "estimate" things instead of just counting them for real?

Q. Why does it use these "hash functions"? (Is it for randomness? Or...?)

Q. What is this "log-log" part in its name supposed to mean? Is there a double logarithm somewhere?

Q. How accurate is it, really? And how does it balance between accuracy and memory usage?

Thanks in advance, and sorry if my questions are noobish.