I'm glad you asked!
All Might is more than a powerful hero. He's the top hero in his world, gifted with the ability to gain a form that gives him super strength, super speed, incredible endurance - the basic superhuman traits. He's the number one hero in a world where mutants are the rule, not the exception. But it isn't just his incredible feats of strength, his ability to tank hits that would leave lesser heroes crumbling into dust, or his ability to move faster than the eye can see that makes him that. It's his unbreakable will and optimism and the power to go beyond that always carries him to victory.
I'll talk about that in a bit, because I think it is essential to this fight. but first, let's discuss a tiny bit about how All Might works.
All Might's power is one transferred from carrier to carrier, unlike most other superpowers which are received at birth. To control All Might's power, the mighty One For All, it takes years of dedicated training. When given to someone it doesn't manifest in the same way each time, but it is still characterized by incredible stregnth, speed, and stamina. The next in line to get One For All managed to unlock a fraction of the potential it gave and he still managed to completely destroy a robot that was specifically created to stop superheroes during their entrance exams from doing anything too risky. It was supposed to be impossible to defeat.
So yeah, even untrained, All Might's power in someone's hands can destroy a giant robot, even if it is at the expense of one's body.
All Might's power lasts for about an hour. It used to last for much longer, but due to a loss against a supervillain that left him grievously injured, he can only do it for so long. An hour is all he needs. Here's All Might, with one punch, obliterating a supervillain that was slowly ingesting a young hero. This punch caused a minor tornado to spring up. That's the power of his Detroit Smash.
I should note that he did manage to save both the life of the child being ingested and the one who ran to protect his friend and nearly got flattened.
In fact, after he saved his life, the character in question became a student at the school All Might teaches at. And what do you know, the student got in a spot of bother when villains attacked again, and All Might managed to jump halfway across a building quicker than the eye could see to rescue two heroes in the clutches of a superhuman beast.
And it's that beast I want to talk about, because that beast is Nomu.
Nomu was an abomination created specifically to destroy and kill All Might. It was a massive being with blows just as powerful as the Detroit Smash and speed just as quick as All Might's, but it also had an accelerated healing factor and shock absorption, which meant that it could tank even more damage than All Might could possibly deliver. It was designed to take the absolute strongest hits again and again and again and still be left unharmed. So what did All Might do when faced with a being able to match him at 100%?
Simple. He went beyond.
All Might put 110% into every blow and managed to fight past his one hour time limit to destroy Nomu and send him flying halfway to space. All Might can tank blows with the best of them and deliver incredible counterattacks, and if he's faced with a barrier he'll just rain on it until he can make it through. All Might is the top hero for a reason, and in the end the hero is here to defeat the villain.
So don't worry, kids.
He's here.