Assuming you are asking about the app drawer on a stock Android device, it is a vertical list of all the apps installed on your device. You can access it by tapping the icon in the bottom dock that looks like a grid of dots.
The app drawer is where you'll find all of the apps that are installed on your device. You can scroll through the list to find the app you're looking for, or you can use the search bar at the top of the screen to search for an app by name.
You can also organize your apps into folders in the app drawer. To do this, simply drag one app onto another and they will be grouped together in a folder. You can give each folder a name to help keep things organized.