What it measures
Book value per share is shareholders' equity divided by the total number of shares outstanding. Where shareholders' equity answers "what is the whole company worth on paper," book value answers the same question per share, the unit an individual investor actually holds.
It's worth comparing against the market price of the same share. A share trading well above its book value means the market is pricing in more than what's on the balance sheet, future growth, brand, intangibles. A share trading below book value is a genuinely different situation worth understanding, not automatically a bargain and not automatically a warning sign either.
A worked example
A company with 800,000,000 in shareholders' equity and 100,000,000 shares outstanding has a book value of 800,000,000 / 100,000,000 = 8 per share.
How Kobo & Cents uses it
Book value lives in the equity category. It's most useful read alongside the current share price shown elsewhere on the same stock detail page, the comparison is the point, not the number alone.