It is simply the value of the assets shown on the company's balance sheet minus the liabilities. Book value is one way to compare two like businesses, but it generally isn't considered a good measure of absolute value. The store with the higher revenues is presumptively more valuable, although of course a buyer will need to examine carefully what market conditions may change. Book value is less meaningful for businesses that don't have significant hard assets, such as service businesses.
Book value focuses on tangible assets, such as inventory, accounts receivable and equipment.
A service business, such as a consulting firm, may have relatively few tangible assets. The key assets of a service business may be the knowledge of the consultants and the long-term relationships with customers. Even in a business that has significant value in hard assets, tangible assets don't represent all the value. A balance sheet may ignore or underestimate the value of intangible assets, like goodwill, know-how and customer relationships. The value of an asset on a company's books may not be reflective of the actual value.