http://braukaiser.com/wiki/index.php/...If you are just doing your first batch all grain, don't worry about it too much.

Usually you need a mash to be in the range of 5 to 6pH with the optimum being

5.2. This will aid in starch conversion as well as good break in the kettle and a clearer final beer.

If you want a more scientific explanation then you can do some reading about it in the link I have provided.