I am not really sure what it means but I like to pretend I do so I seem smart.
A histogram is a small chart that can be shown on the camera monitor. It allows you to better judge your exposure.
That said, When I'm shooting out in the field, I check the histo first. If it looks good, then I'll check composition and focus. My monitor is set up to look like this:
If I'm shooting in very consistent lighting, then I'll forgo chimping the histo. As soon as I detect a lighting change, then it's back to checking the histo.
The only time I don't normally watch the histo is when I'm shooting in a controlled lighting situation (i.e., studio work).