SLIME's cross-reference commands are based on the support provided by the Lisp system, which varies widely between Lisps. For systems with no built-in XREF support SLIME queries a portable XREF package, which is taken from the CMU AI Repository and bundled with SLIME.
Each command operates on the symbol at point, or prompts if there is none. With a prefix argument they always prompt. You can either enter the key bindings as shown here or with the control modified on the last key, See Key bindings.
There are also “List callers/callees” commands. These operate by rummaging through function objects on the heap at a low-level to discover the call graph. They are only available with some Lisp systems, and are most useful as a fallback when precise XREF information is unavailable.