When you first start training it's easy to make progress. You can do more reps, lift more weight, run further, run faster, any kind of progress at all – just by showing up and training.
After a while though, that stops. No matter how hard you try you just can't seem to do any better. You've hit a plateau.
This isn't because you're doing anything wrong.
This is how the body works. It will adapt to a new stimulus for about 4 – 8 weeks. It will become faster, stronger, muscle size will increase, you'll produce more red blood cells etc, depending on the kind of training.
But after 4 – 8 weeks that's it, adaptation stops. No more adaptation, no more progress.
How do you keep moving forwards? By changing your training, and giving your body something new to adapt to. It doesn't have to be a big change either. Altering pretty much any variable will do.
This is often done by changing reps and sets. For example, by doing fewer reps and sets with a heavier weight, or doing more reps and sets with a lighter weight, but changing any variable will do.
For instance, one method that gets overlooked is that you can keep the reps, sets and weight the same but take off a little rest time in each successive session in between the sets, before eventually returning to the original rest time but with a heavier weight, or doing more reps or more sets.
For example, in the first week you might do a set and then wait 60 seconds before doing the second set.
The next week you might do the first set and then wait only 50 seconds before doing the next set.
The week after, you'd then do the first set and then wait only 40 seconds before doing the next set, and so on, until eventually you start back again at the original rest time, in this case 60 seconds, but with a heavier weight than you could handle before.