Okay little one, imagine you want to order food from a restaurant. FastCGI is like a waiter who takes your order and goes back to the kitchen to get your food.
When a website uses FastCGI, it means that the website asks FastCGI (the waiter) to deliver a page to a visitor. FastCGI then takes that request (the order) and goes back to the server (the kitchen) to get the page.
The server might take a little time to cook up the page, but FastCGI can handle other requests (orders) while it waits. Once the page is ready, FastCGI (the waiter) brings it back to the website (the customer).
This process is faster because it allows the server to handle multiple requests at once, just like a waiter can handle many different tables in a restaurant. So, FastCGI helps make websites run faster and more efficiently.