I'll try to dumb it down a little for you.
A Linux distribution is an operating system, like Windows or OS X are. An operating system is basically software that lets a computer be useful to a person. Without an operating system, a computer is little more than just taking up room on your desk.
Linux itself, however, is not an operating system. Linux is a kernel, and you can think of that as maybe a conductor in an orchestra. Without someone directing all of the instruments, they can not work in harmony and would just make noise. The same is true for a computer. You have a lot of different individual pieces of hardware in the computer. You also have software running on the computer and the software needs to interact with the hardware. So, the kernel makes sure that all of the software and all of the hardware can all communicate with each other in a coherent way. Things "talk" when it is their turn to talk and stay quiet when it is not their turn.
The kernel by itself is not very useful for you. You need software to make the computer useful. So, if you take a kernel and combine it with a lot of software, you end up with an operating system, such as a Linux distribution like Ubuntu or Fedora.
Some might say I dumbed this down way too much, but I was trying to put things in terms that would make sense to someone new to the workings of a computer.