When you use a computer, you store information on different parts of it like the hard drive. Think of the hard drive like a bookshelf with many different books on it. Each book has a title, and you can find it by looking for the title.
When you connect a new device to your computer, like a USB drive or an external hard drive, the computer needs to give it a place on the bookshelf. This is called drive letter assignment.
Think of it like giving a new friend their own shelf in your room to put their books on. You could label it with a letter, like "A" or "B". Your computer does the same thing with drives that you attach to it, like a USB drive. It gives each one its own letter to find it easily later.
So, when you plug in a new USB drive, your computer might label it with a letter like "E:". That means you can easily find it later by looking for the letter "E" when you want to access it. This makes it easy to find and use different drives when you're working on your computer.