If you play Fate/Grand Order, there's a chance that someone mentioned stars to you at some point or told you that you have to give certain servants stars to get them to hit hard enough to do some significant damage. If you're like me and pretended to know what critical stars were so none of your fellow players would make fun of you, don't worry, we're here to break it down for you. You'll be an expert Master in no time!
Now's The Time For "Quick" Thinking
Your best friends for using this are the servants who use Quick card damage; think Astolfo, Caesar, Hassan of the Cursed Arm, Atalante, Marie Antoinette, Ushiwakamaru. These units drop the stars as they hit the enemy, filling up that gather rate bar at the bottom of the screen on the right. The more you collect of these in one shot, the more damage the servants will do in the next turn. You'll notice as you accumulate that it will say 'critical' about servant cards, meaning they're going to do extra damage. Certain servants also possess a skill called the Critical Star Gather Rate which allows them to accumulate extra stars with Quick cards when it's activated.
Don't Be Stingy With Those Stars
There are several non-quick servants who, when put into a team with the ones that collect stars, do their maximum damage. These include Saber Lancelot, Archer Emiya, Jaguar Warrior, Berzerker Lancelot, and Nursery Rhyme. Put these units in teams with the servants that generate stars; normally, you should make sure you don't put two of the same servants who absorb stars in a team together, but Cursed Arm as a master star printer can help with this. He's a 2* rarity so you should have no issues getting him at all from the Friend Point gacha. Training him should be pretty fast and once you do that, you can do his recent rank up that has made him the best star-maker in the game; some lower-ranking servants stop being useful when you get to a certain point in the series, but you can think of Cursed Arm as the assassin who never goes out of style.
Hopefully, this cleared up how to use stars for you, and may you be blessed by the luck of the gacha!