Resistance training (also called strength training) is any exercise that makes your muscles work against a load: free weights, machines, resistance bands, or your own body weight. The principle that makes it work is progressive overload — gradually asking a little more of your muscles over time, by adding weight, reps, or sets, or improving range of motion — so the body has a reason to hold onto and build muscle rather than let it go.
It's the single most important form of exercise on a GLP-1. Rapid weight loss on these medications takes lean tissue along with fat unless you actively defend it, and resistance training is the signal that tells your body to keep muscle while fat comes off; paired with adequate protein, it's the combination that works. Two or more sessions a week covering all the major muscle groups is the usual floor. See why strength training matters on a GLP-1 and a beginner's resistance-training plan.