Okay kiddo, let’s say you have a big bag of candy and you want to know if there is more red candy in the bag than green candy. The idea that there is more red candy is called the alternative hypothesis. It's like saying "hey, maybe there's more red candy than green candy in this bag."
The opposite of the alternative hypothesis is the null hypothesis, which is the idea that there is no difference between the amount of red candy and the amount of green candy in the bag. It's like saying "hey, maybe there's the same amount of red candy and green candy in this bag."
So when you’re trying to figure out if there is more red candy in the bag than green candy, you come up with an alternative hypothesis (maybe there is more red candy), and then you do some testing to see if your alternative hypothesis is true.
If your testing shows that there really is more red candy in the bag than green candy, then you can accept your alternative hypothesis and say that you were right all along. But if your testing shows that there is no difference between the amount of red candy and the amount of green candy, then you have to reject your alternative hypothesis and say that there is no evidence to support it.
Does that make sense?