A specific number you track regularly to see whether your business or app is moving toward its goal, such as weekly sales or return rate.
KPIs are the few numbers that truly reflect the health of your business. For an online store they might be order count, average order value and conversion rate; for a subscription app, active subscribers and churn. What matters is that they tie to a clear goal and can be measured.
Without clear KPIs, you build, launch and spend without knowing whether it works. With so many numbers in analytics dashboards, it is easy to get distracted by vanity metrics that look good but mean little, such as app downloads without real usage. Practical tip: pick just three KPIs to start and put them on a simple dashboard you review every week. Ask of each number: if it changed, would I change my decision? If not, it is not a key indicator.