There's a fundamental concept about direct and indirect band gap semiconductor that I am not clear about
Given below is the image of the direct and indirect band gaps from Kittel
The y axis represents energy. But I am a bit confused about the x axis. What that K is actually? All that I can make out is that its some kind of wave vector.
Below is an image from wikipedia
It shows the x axis as momentum. The accompanying description states this as crystal momentum. On looking up the related pg again from wikipedia
I would like to know what exactly this value on the x axis represents? Why is it taken on the y axis? And what does the variation of energy with k actually signifies?