Humans have a multitude of resources at their disposal, one crucial one being time. Thinking about the value of time is the deliberate action of estimating the worth of one's unit of time, comparing it to other resources at one's disposal, and then being able to make explicit trades between time and other resources.
This is often used in relation to productivity and optimization. Estimating the value of one's time enables further concrete actions, such as: how can you spend money to free up time? how much should you be willing to spend to free up one hour of your time? what's the value of an additional work hour for you versus an additional hour of leisure?