A football field is a big piece of grass where people play a game called football. It looks like a rectangle, and it's about 100 yards long and 50 yards wide.
A yard is a unit of length that measures how far something is. It's like a very big step. So, if you take 100 of those big steps, it's the same as the length of a football field.
Now, imagine you want to measure the distance between two places. You can use yards to measure it. For instance, if those two places are as far apart as the length of a football field, you say they are 100 yards away from each other. If they are only as long as half the football field, you say they are 50 yards away.
It's important to know the length of a football field so that people can play the game in a way that's fair and consistent. That's why it's a standard measurement in football.