A web application is like a toy box filled with toys that you can play with using your computer, tablet or phone. Just like how your toy box is always there with you, these web applications are always there when you open them up on the internet.
To use a web application, you go to a website, just like how you go to a playground to play with your toys. Once you enter the website, you can interact with the different features the web application provides. These features could be like playing a game, drawing pictures, or even checking your email.
For example, imagine playing a game online. The game is not actually on your computer, it is on the internet, but you can play it on your computer like it's on your computer. You can use your mouse or keyboard to interact with the game and it responds, just like when you play with toys that respond to your actions.
Web applications allow people all around the world to use them from anywhere they have internet, like playing a game with someone on the other side of the earth. They also make it possible for people to store information and data on the internet, like keeping track of your toy inventory.
In short, web applications are like virtual toy boxes that you can play with on the internet.