OP here, apologies for the lack of clarity. I mean to say that a lot of people will go to the gym to do "weight training" but will only use dumbells or machine

OP here, apologies for the lack of clarity. I mean to say that a lot of people will go to the gym to do "weight training" but will only use dumbells or machines, etc, at such low weights that the effort is mostly futile in terms of developing your muscles. A good rule of thumb to go by is to use the maximum weight which you can successfully complete 8 - 12 repetitions of the motion without sacrificing good form (bad form will lead to injuries). edit: in response to a query of yours below, don't worry about accidently gaining strength if it is not your goal, it takes focused training to go up in strength. If you just want to maintain current muscle you just need to keep doing lifts with the weight you can manage 12 reps with. if you don't push to increase the weight from that point you will plateau at that point