Why Does Infinity Have Different Sizes?

Imagine you have two bags of marbles. One bag has all the red marbles, and the other has all the blue marbles. If you can match each red marble to a blue one perfectly, they both have the same number of marbles, even if it looks like there are way more in one bag! This is like how some infinities are the same size even though they seem different. But when you get to something like all the numbers between 0 and 1, that infinity is bigger than the one with just whole numbers.