Ad serving is like a waiter bringing food to your table at a restaurant. Ads are like the food that the waiter brings to you. When you go to a website or app, you might notice ads popping up on the screen. Ad serving is when a website or app puts these ads on their page so you can see them.
Think of it like this: when you go to a restaurant, you order your food (or ads!), and the kitchen (or ad server) makes it for you. The waiter (or website/app) then brings your food (or ads!) to your table for you to see. The process of the kitchen making your food and the waiter bringing it to you is similar to the website or app providing space for ads to show up and for the ad server to place the ads.
Ad serving is important because it helps websites and apps make money. Companies pay to have their ads shown, and the website or app gets paid for showing them. Just like how the waiter gets a tip for bringing your food, the website or app gets paid for showing the ads to you.