Weight in fitness = lean weight (muscle, bone, etc. )+fat weight. Thin people can gain weight by gaining muscle. You just went to the gym and gained muscle. Obviously, you have gained weight. Later, it may be due to insufficient training or improper training methods that the muscle gain is not obvious.

Muscles can be trained to increase volume, strength and endurance. Different training methods have different effects on muscle exercise. 1~6RM mainly develops muscle strength. 6~ 12RM mainly develops muscle mass. The muscle endurance is developed with the strength less than or equal to 12RM.

RM: maximum number of repetitions. A barbell of 100 kg can only be lifted twice with all its strength, so the weight of 100 kg is 2RM for you. It varies from person to person.