Strength: It increases damage when you punch, the amount it increases is 1 every 3,500 strength. Strength also affects the damage of melee skills.
Speed: This increases your movement speed but there are diminishing returns, the movement speed gained from going from 1 million stats, to 10 million, is similar to the speed increase from 10 million to 100 million.
Defence: You have 100 HP by default, your total HP is 1% of your defense +100
Energy: Increases your energy bar by 1/500 of your energy stat or 0.2%, it also affects the damage of energy skills. Your energy will always regenerate on a percentage of your max energy, this can be affected by the energy charge game-pass.
Note: While Punching, flying, sit-ups, and shooting energy attacks increase your stats, the best way to get stats is to simply complete quests. This will ensure all stats remain balanced and the stat gain is significantly higher anyways, especially against later bosses.