The term refers to a specific, widely circulated software patch designed to bypass the licensing verification of Internet Download Manager versions 6.xx (e.g., 6.38, 6.40, 6.42, etc.). The "v21" denotes the patcher's own version number (likely the 21st iteration of that specific crack), and "Top" is a marketing tag used by crackers to suggest superiority over other patches (such as "IDM Activator" or "IDM Crack").
IDM 6.xx Patcher v21 (often termed "top" in search contexts) is idm 6xx patcher v21 top