Yep you have probably heard of it, but what does it actually mean.
Well in simple terms any framed image (art or photo) is broken into thirds along the horizontal & vertical edges. By 'framing' in photography means the image you have framed to capture in your photograph.
As per the sample below you can see how the image is broken into thirds. The idea is to align your key components of the image to a third, like the cyclists below.
By using the thirds you can see how it gives the image some character. It's not to say don't have a subject in the centre, but using the thirds rule helps give interest to what could be a plain image.