Someone/Something who does one specific thing extremely well, and noticeably doesn't do other things as well. Usually the "other things" are similar/adjacent things.
"Max is a one-trick pony at cooking; his pasta dishes are scrumptious, but he can't cook anything else to save his life." "This frying pan is a one-trick pony; it can cook eggs to perfection, but will usually burn anything else you put on it." "He's a one-trick pony at karaoke; he's a master at singing rock songs, and that's about it." (This implies he can't sing songs of any other genre as well)