Okay kiddo, do you know what a thoroughbred is? It's a special type of horse that is bred to run really fast. Now, people who breed thoroughbreds have different theories on how to make the best and fastest horse.
Some people believe in something called "line breeding." That means they try to breed horses that have similar ancestors or "bloodlines" together. They think that this will make the offspring better because they will have inherited certain traits from their ancestors that make them good runners.
Other people think that it's better to have a mix of different bloodlines. They think that if you breed horses that have different strengths and weaknesses together, you'll end up with a horse that is well-rounded and can run well in different types of races.
There are also different theories on what traits are most important to breed for. Some people think that stamina is the most important, because a horse that can run for a long time without getting tired will have an advantage in longer races. Others think that speed is the most important, because a horse that can run really fast will have an advantage in shorter races.
So, when people talk about thoroughbred breeding theories, they're really talking about different ideas on how to make the best horses. Some people think you should breed horses with similar ancestors, others think you should mix it up, and there are different opinions on what traits are most important to breed for. It's kind of like cooking - there's more than one way to make a delicious meal!