Book value is what the accounts say the shareholders own outright. Price to book compares the market value of the equity against that figure: below one, the market is valuing the company at less than its stated net assets.
The measure is most meaningful for banks and insurers, where assets are largely financial and marked close to their market value. For a software company it is nearly useless, because the brand, the code and the customer relationships that produce the earnings are barely on the balance sheet at all.
A low price to book is not automatically cheap. It frequently means the market disbelieves the carrying values, expects impairments, or sees a business earning less on its assets than they cost to maintain.