An online mouse jiggler is a browser-based mouse jiggler: a web page that tells your operating system to leave the display on. Hardware and software versions fake mouse movement, but a web-based mouse mover cannot touch your real cursor, because browsers block that. It asks for a power management wake instead, so the monitor stays lit and the screen saver or screensaver stays off. Any mouse jiggler built this way works like a polite request, not a trick.
That makes it a mouse movement simulator in effect only: your screen sees constant activity, though your chat app may not. It is free, needs no installation, and you never have to install anything or touch admin settings. Close the tab and your normal behavior returns. Do not expect an online mouse jiggler to hold an active status in every app, since the cursor never really moves. A free mouse jiggler that claims otherwise is overselling.