What is pyramid training?
Pyramid training is when you increase the weight of the exercise and decrease repetitions.
1st series x 15 repetitions
2nd series x 12 repetitions
3rd series x 8 repetitions
4 series x 6 repetitions
Or 4 series x 20-15-10-8
Following this scheme, you started with a lighter weight in the first grade, because you would perform more repetitions.
As you advanced in the series, you would decrease the repetitions (15 -> 12 -> 8 -> 6) and increase the weight (example: 60kg ->70kg -> 80kg -> 90kg -> 100kg), since performing fewer repetitions per series would allow you to do so.
This is an example of an exercise using the pyramid training method.