'Time is of the essence' is a phrase used to emphasize that previously described actions need to be completed in a reasonable amount of time. It is often used in contracts where time and dates are stipulated for the completion of certain actions and any delay could be grounds for cancelling the agreement. In everyday English we use it to mean it is important to do intended actions as quickly as possible otherwise there may be problems.

Time is important / Time is of value / Don't waste time

What will italki do without you Eliot :)

God knows!