A reason to act now instead of later. Usually time-based.
Without urgency, prospects procrastinate and never buy. Urgency is any reason the decision needs to happen now: a deadline, a countdown timer, a 'this week only' sale. Urgency works because humans have infinite future-self optimism and finite attention today. If they can buy later, they'll buy never.
Don't fake urgency; customers sense it and lose trust. But if you have real urgency (a cohort starts; the price goes up; bonuses expire), USE IT. It's the difference between a prospect thinking 'I'll come back to this' (and never coming back) and 'I need to decide today.'