I was wondering too because of my old account, I thought this meant what rank you rating is compared to everyone elses rating, looking at the graph lichess can show you.
The word rank is very literal in its meaning here. Its the same as what you infer when i say that you stand 10th in your class. mathematically, { [ total number of players - ( total number of players * how better your rating is compared to others(in %) ) ] + 1 }.
Your rank is your position in the entire Lichess community in a particular variant.
