What breaks a concentration spell?

You lose Concentration on a spell if you cast another spell that requires Concentration. You can’t concentrate on two Spells at once. Taking Damage. Whenever you take damage while you are concentrating on a spell, you must make a Constitution saving throw to maintain your Concentration.

https://roll20.net/compendium/dnd5e/Spells#:~:text=You%20lose%20Concentration%20on%20a,throw%20to%20maintain%20your%20Concentration.