A portable application is like a toy that you can carry with you wherever you go. It's like having your favorite toy in a nice box that you can easily move from one place to another.
In grown-up terms, it's a computer program that you can store on a USB drive or other portable device, so you don't have to install it on a computer to use it. It doesn't take up much space on your computer, and you can run it on any computer you have access to.
Just like with your toy box, you can fill your portable device with lots of fun applications, like games, music players, or even word processors. Then, when you need to use them, you plug your toy box (or portable device) into the computer and start playing with your favorite toys (or applications).
In summary, a portable application is like a toy box that you can carry with you, filled with fun applications that you can use on any computer without having to install them.