Churn is the rate at which existing customers stop paying — canceling a subscription or simply not renewing. It is usually stated as a monthly or annual percentage of customers or revenue. In SaaS, churn matters more than almost anything else because it eats MRR and ARR faster than new sales can replace it. A company growing 10% a month but churning 9% is basically running to stand still.
In plain English
Churn is the leak in the bucket. You add customers at the top and lose them out the bottom. Low churn means your forecast compounds; high churn means the sales team is always filling a hole instead of growing.
Why it matters to you
Churn explains why your quota feels harder than it looks and why expansion and renewals are so prized. When you are selling, the buyers know their own churn problem — addressing it is a strong value proposition. Asking about a company's churn in an interview tells you whether you are joining a growth engine or a leaky boat. Sales enablement is often the tool teams use to attack it.