Here's the histogram before any adjustments:

The bottom axis is brightness, black to white from left to right. The height axis shows what proportion of the picture is that brightness. So this shows that there are only light tones, no solid blacks or even really dark greys in the picture. This is normally a bad thing. (Except when it's not, of course - that's  a judgement to make.)

So, we use auto-levels and get this:

Now there are areas of every brightness in the image, which normally looks more natural: our eyes adjust in roughly that way. How you adjust the levels depends on what tool you're using of course - you can fine tune it with a lot of them.