Causal filter

Causal filters are like an invisible wall that stops some things from passing through it. It's like standing in front of a wall, and the wall is made out of something that will stop some things (like a ball, for example) and let other things (like air) pass through. Imagine that you had an invisible wall made out of a material that makes it so that only certain frequencies of sound waves could pass through it. Those sounds that can pass through the wall are called causal sounds, and the wall that stops others is a causal filter.