Maximal means the greatest, best, or largest amount of something. The maximal speed of your friend's junky car might be forty miles per hour.
The adjective maximal is the opposite of the more common minimal. Your idea of sleeping in maximal comfort might include a soft comforter and lots of pillows, and you might try, when you're running on a treadmill, to get your heartbeat to its maximal rate for several minutes. Maximal has a Latin root, maximus, or "greatest," which in turn comes from magnus, "great or large."