Creatine is a compound your body makes and also gets from meat and fish, stored mostly in muscle where it helps regenerate the quick energy used for short, hard efforts like a set of squats. Supplementing with creatine monohydrate (usually 3–5 g a day) tops up those stores, and it is among the most thoroughly studied sports supplements, with a long track record for supporting strength and training performance in healthy adults.
On a GLP-1 it comes up because the goal is to lose fat while keeping muscle, and creatine can help you get more out of resistance training when appetite and energy are low. It's an aid to training, not a substitute for it or for protein. A small early weight gain from extra water in muscle is normal, and anyone with kidney disease should check with their prescriber first. See supplements on a GLP-1 and why strength training matters.