An ace is a serve your opponent never even touches — instant point. A double fault is two missed serves in a row: the point goes to your opponent without them hitting a ball.
Both terms hang on the second serve, and more is decided there than beginners suspect. Hit your second as hard as your first and you collect double faults; merely push it in and you get attacked immediately. The way out is spin rather than pace. A kick serve clears the net in a high arc, dives steeply and kicks up towards the backhand — that cuts your double-fault rate noticeably without gift-wrapping the ball. The slice serve is the easier one to learn, since it asks for less back extension and shoulder rotation, and many coaches build it first. One expectation also needs correcting: aces are rare at amateur level and are not a sensible target. A safe, well-placed serve that lets you attack the next ball wins you far more points across a full match than any risky swing for a winner.