Okay kiddo, do you know what a sign is? It's a big thing with words or pictures on it that helps you know what's around you. An advertising board is kind of like a sign, but it's used to show people information about products or services they can buy.
Advertising boards can be all different shapes and sizes, and they usually have bright colors and pictures on them to catch people's attention. They might be outside a store or on the side of a road to let people know about something they can buy.
Sometimes, companies pay money to put their message on an advertising board because they want people to know about their product or service. For example, a fast food restaurant might advertise a new burger they have, and people passing by will see the ad and maybe decide to go try it.
So that's what an advertising board is, a big sign that helps companies tell people about what they have to sell.