Book value is what the accountants say is left for equity holders. It is backward looking and uses historical cost for many assets, so it understates companies whose value is brands, software, or people, and it overstates companies carrying stale inventory or goodwill from a bad acquisition.
Traders use it mainly as a reference in financials and asset-heavy sectors, through price-to-book, and as a rough floor in distressed situations, though chapter-11 usually proves the floor is not real.
Example: assets of $9.0B and liabilities of $6.6B give book value of $2.4B. On 300M shares that is $8.00 of book value per share. At a $24 share price the stock trades at three times book.
Related: price-to-book, book-value-per-share, par-value, enterprise-value