We become stronger when our muscles move more load than they previously could. Progressive overload is the key to muscle training. It starts with bodyweight exercises like squats or pushups, and gradually incorporates additional weight such as barbells, kettlebells, dumbbells, and slam balls.

At the end of the day, our muscles don't know how many kilos we have lifted or how many reps they have done. You can use a combination of increased weight or increased reps, or often decreased reps, but stepping up the weight to keep your workouts time effective. 

Our muscles respond to an increase in what they have done before. A really simple way to measure this is if you become sore after the workout, commonly known as DOMS (delayed onset muscle soreness). 

When you become sore, it's safe to say that we have achieved overload. During this phase, active recovery is recommended (don't just sit, walk and move, maybe a ligher session which will promote blood flow and aid your recovery), eat well to aid recovery, and be excited that you are taking steps to becoming stronger.