Lichess is a web-based chess program that you play from within your browser. You can play against an AI or other people and, amongst other things, you can set the difficulty level when playing against the AI.
That aside, there’s nothing much to explain apart from where it is. For the rules of chess, we’re not the right folk to ask!
Oh, one thing. The interface is in Gaelic but most players speak English. But that doesn’t mean you couldn’t arrange a game with a friend who speaks Gaelic.