I enjoyed this clear description:

Meditation is basically a training method for your mind. When certain things happen to you, your mind generates a certain response whether it be happiness, frustration, anger ect. The way your mind has been inculcated is the path of least resistance and the path it wants to take, and will take unless you know how to mitigate it. Meditation teaches you how and makes it easier to override the process. Who is doing the overriding of this process? Well, that’s the million dollar question. But I digress.

So here’s what I’m getting at: if meditation is too easy, you’re doing something wrong. You might be getting yourself really relaxed, but is it possible that’s all you’re doing? Not saying it is. I don’t know, just throwing some ideas out there and it’s up to you to see if any seem to fit your situation.

But as you meditate, your mind wants to grab onto the thoughts and not your breath. The course of least resistance is away from your breath and back into whatever thoughts are vying for your attention. Every time you go back to the breath, you train or teach yourself even, to take the opposite of the path of least resistance. This is coupled with the fact that half the time when you meditate, your mind says, “I’m tired. Stop concentrating on the breath and just kick back and let a guided meditation do most of the work.” But every time this comes up you learn to drop it by returning to the breath and not listening to the thought no matter how loud and powerful it can get.

When you first start meditating you have this thought and then come back to the breath. But there’s still a trace of this thought floating around in your mind and eventually it pulls you in again. As soon as you realize your back in that thought again, you turn your awareness back to the breath and away from the thought. But then it pulls you in again. And then you drop it again. You do this over and over and over. But as you practice you get better and better and faster and faster at recognizing it. You start to figure out how to do it most efficiently and quickly, seeing and dropping thoughts before they even become thoughts at all.

After doing this hour after hour, you gain a skill. One day you realize that you don’t have to be sitting on a cushion to use this skill. I can’t really explain how it’s done, but it’s just something you learn from continually focusing, coming back to, and holding your attention on the breath. It’s like if you ever do a lot of push-ups, eventually you will realize, “I can flex my pecs.” You couldn’t flex them before, and you don’t really know how you learned to do it, but now you can just do it.

(Hat tip to Andy McKenzie)