First let's talk about the original system that Kaga Michio the og protagonist of the story had. As that system is present in this novel with almost no changes.
This system is based on classic rpg games. When you play a rpg game it gives you a character selection page, inside it for now forget about name, race and appearance, focus on the additional things that the characters can have at the start.
In those rpg games you are given extra 'bonus points' at that character creation page which you can invest in anything you want and so you can build a unique character.
For example: In the start you get 10 bonus points, and you can add them to any of these 3 stats. Strength, Agility and MP. So if you would like to make a fighter then you will add more points in strength, if you want an archer you will put the points in agility and if you want a mage you will put the points in MP.
So Michio also got bonus points at the start of his character creation page. He got '99' bonus points or 99 BP for short. Then he added them to his character in different things which we will get into later, but one of those points he added into an overpowered skill which changed everything. That skill was "Character Reset".
In all these rpg games before the game starts you are allowed to 'reset' the character points or mainly you are allowed to add or remove them from any stat as much as you want as the game hasn't started yet and so you are allowed to fool around. You can't change them once you confirm those changes and start the game.
But character reset allows you to change these Bonus points and reallocate them to wherever you want even inside the game. It's like a cheat code basically.
For example: You played the game from the previous example and you added all the 10 bonus points into Strength, so you are playing as a melee fighter. But after you are playing for a while you feel like you are not having that much fun so you use the cheat code 'Character Reset' and remove all the 10 points from strength and add them to MP, now you can play as a wizard and you don't have to restart the game or make a new character.
So Michio added 1 BP from his 99 BP to this skill 'Character Reset', so that he can change his BP around inside this system whenever he wants after getting isekai'd. As long as that 1 BP is added to character reset skill, he can fool around with BP as much as he wants, it's like having a character creation sheet open all the time in the real world.