If you just want to know how to type it, the REPL help tells you:
help?> ∂
"∂" can be typed by \partial<tab>
This kind of tab completion is supported in the REPL and can also be configured in most modern editors.
But by default, ∂ doesn’t have any definition in Julia, and is just another possible identifier, like x or f. It is up to you to assign a meaning to it.