HTML5 is the current standard for building web content, and it is the technology modern online slots use. A game built in HTML5 runs directly in a browser on desktop, phone or tablet, without plug-ins.
Earlier online slots relied largely on Adobe Flash, which did not work on most phones. The industry moved to HTML5 so the same game could run everywhere, which made mobile slots practical. Many older games were rebuilt in HTML5 during that move, sometimes with changes to their look.
For players, the format is invisible. The game's result is still decided on the provider's server by a certified RNG; the browser only shows it.