With stock options, this is a measure of the financial benefit of exercising options at the end of the term instead of exercising earlier. You can think of it as the value of having an option to purchase until the end of the term, when the stock price and spread may be higher, instead of exercising today. Various option-valuation models try to calculate options' time value. See a related FAQ
Further reading about stock option valuation is available in the section Basics: Valuation & Expensing elsewhere on this website.