Filtering your items allows you select a group of items, and any item not in the filter will be hidden from view. Filters stay on until you clear the filter or you leave the Pricebook. You can tell if your grid is filtered by looking at the bottom of the My Pricebook window. Filters will be used for doing things in a batch. The most common use will be to do group price changes.
To view the Filter tool click the Filter button from the Pricebook screen.