Do you know what a sieve is? It's a tool with tiny little holes that lets small things through, but catches big things. Filter theory is kinda like that, but for people and their relationships with each other.
People use filters to decide who they spend time with and who they don't. These filters help them find people who have things in common, like interests or values. Filters can be things like where someone lives, what school they go to, or what job they have.
Sometimes filters can prevent people from meeting others who are different from them. For example, if someone only hangs out with people from the same social class or race, they might miss out on learning about other cultures and ways of living.
Filters can also change with time. Someone might have different filters when they're a kid, teenager, or adult. Filters can also be influenced by the people around us, like our family and friends.
Overall, filter theory is about how people use different criteria to decide who they want to be friends with or who they want to spend time with. It's like a way of finding people that are similar to us, but at the same time it can limit us from meeting those who are different from us.