Active insulin time (AIT) refers to the time insulin remains working in your body. It is a safety feature mainly used in insulin pump therapy to help prevent over-correction of high glucose levels. Also known as Duration of Insulin Action (DIA), AIT is also used in bolus calculators to prevent insulin stacking (taking insulin injections too close together).
DIA or AIT can be set for a time up to 4.5 hours, however, in most cases, 3 hours is used.
Knowing how much insulin is on board will help you get a more accurate insulin dosage, and will thereby improve your time in range.
If you are using an insulin pump you can see if you have any insulin on board from your status screen. It is a little trickier to establish any active insulin if you are on multiple daily injections.