Power level, is a number displaying how powerful you are. The higher the number, the more powerful you are. And like said before, the higher it is, the more powerful you are, which means you have more energy, stronger, can take more damage and so on.
How to increase your Power Level:
- Battle
- Quests
- Sagas
- Roleplaying
- Training
How do you know how powerful your opponet is? You could sense their ki ( You need to learn it ), or use a Scouter ( Most scouters have Power Level limits, if it passes the limit, it breaks ).