It is a technique of increasing the difficulty of the exercise you are doing that helps you with making sure that you build your muscles effectively and efficiently. The difficulty can be increased by increasing the weight or the number of reps.
Example
Let’s take bench press as an example where you do 5-8 reps.
During one day you do 6,5,5 reps and next time you do 7,7,5. This is still progressive overload because you’ve increased the number of reps within 2 sets. You do it until you reach 8 reps and then you increase the weight.
You may reach weight’s plateau but you still need to progressively overload. You can do it by decreasing the rest time between sets or slowing down the exercises and pausing