Asymptotic complexity is like when you have to count a lot of things and you want to know how long it will take you. Imagine you have a big pile of toys you need to sort into different groups. You can do it one toy at a time, or you can use a machine that can sort several toys at once.
Asymptotic complexity is like figuring out how many toys you can sort in a given amount of time. If you use the machine, you can sort more toys faster, but there is still a limit to how many toys you can sort in a given time.
When we talk about asymptotic complexity, we are trying to figure out how long it will take to sort a certain number of toys using different methods. The method that takes the least amount of time is the most efficient.
But sometimes, we have so many toys that even the most efficient method will take a long time. That's why we use asymptotic complexity to describe how a method will behave as the number of toys gets really big.
It's kind of like if you had a million toys to sort. It would take a really long time no matter which method you used. But if you just looked at how long it took to sort, say, 10 toys, that wouldn't really tell you much about how efficient a method is.
So, asymptotic complexity helps us predict how long it will take to sort lots and lots of toys, even if we don't actually have that many toys to sort right now. And that helps us choose the most efficient method for sorting toys (or doing other things that involve lots of steps).