Cached Input - Cost & General API Use.

Can someone please explain this to me, I'm new to actual API use and cosumption. I'm building a small app that uses a plathora of models, including gpt-5. But the way I have it coded right now is very very very expensive. I'm sending all the context window of the whole chat at each request.

It's basically a chatapp but in the studying niche.

Yesterday I found out about Cached Input, I did some research on my own but I'm still very confused.

Can you explain Cached Input to me? Also any advice on context management would be greatly appreciated.

tl;dr: What's cached input?