Play Online connects to Lichess and plays real games against real opponents through your own Lichess account. Games are seeked, played and rated on Lichess; the board here is a client for it. Once a game finishes it can go straight into analysis, which is the part that makes playing useful rather than just time spent.
Time control changes what a game trains, and the choice matters more than most players assume.
Rapid — ten minutes or more per side. Long enough to calculate a line properly and notice what your opponent threatened. If you are trying to improve, this is the format that rewards it, because it gives you time to apply anything you have practised.
Blitz — roughly three to five minutes. Fast enough to force decisions on recognition rather than calculation. Useful for openings and for building pattern speed, less useful for learning to think, because there is rarely time to.
Bullet — under three minutes. Primarily a speed and reflex exercise. Enjoyable, but it will not teach you much about chess, and a long bullet session will teach you to move before you have looked.
A common pattern worth avoiding: playing fast games to get more games in, then finding the games were too fast to learn anything from. Fewer, slower games with a review afterwards produce more improvement than a larger number of quick ones.
The habit that separates players who improve from players who plateau is reviewing games they have just played — particularly the ones they won, which usually contain mistakes that went unpunished.
Run a finished game through Game Report, which analyses every move with Stockfish in your browser and marks where the evaluation moved. Look at the largest swing first and work out what you would have needed to see. If the same kind of error keeps appearing across games, a Profile Report analyses a batch of recent games and reports which weaknesses actually cost you rating, and mistake training turns your own missed positions into puzzles to repeat.
Games play through the Lichess Board API using your account, so results, ratings and game history live on Lichess exactly as if you had played there directly. Lichess is free and open source, and an account is required to play — there is no anonymous play here.