Imagine you have a lot of money and you want to count it. Instead of saying one thousand, two thousand, three thousand, etc., we can use a special system to make it easier.
The long scale is when we use the words million, billion, trillion, etc. For example, one million is 1,000,000 and one billion is 1,000,000,000.
The short scale is when we use the words thousand, million, billion, etc. For example, one million is 1,000,000 but one billion is 1,000,000,000.
The difference between the two is how many zeros are in each number. In the long scale, each new word means one million more zeros. In the short scale, each new word means three more zeros.
Some countries use the long scale and others use the short scale. It's important to know which one is being used so you can understand big numbers better.