It’s a convention used in other Lisps, such as Common Lisp, to distinguish between special variables, as distinct from lexical variables. A special or dynamic variable has its binding stored in a dynamic environment, meaning that its current value as visible to any point in the code depends upon how it may have been bound higher up the call stack, as opposed to being dependent only on the most local lexical binding form (such as let or defn).
Note that in his book Let Over Lambda, Doug Hoyte argues against the “earmuffs” asterix convention for naming special variables. He uses an unusual macro style that makes reference to free variables, and he prefers not to commit to or distinguish whether those symbols will eventually refer to lexical or dynamic variables.
Though targeted specifically at Common Lisp, you might enjoy Ron Garret’s essay The Idiot’s Guide to Special Variables. Much of it can still apply to Clojure.