A key performance indicator (KPI) is a metric that helps you understand how you are doing against your objectives. Create a list of KPIs to monitor and optimize. We shouldn't select more than 3 KPIs (one is optimum) in parallel and always we need not create conflicting KPIs. For example, if driving traffic is important, driving sales is not possible to become equally important. To elaborate here, to get the traffic KPI you may have to “open up” a bit of the funnel to see who would click on your banners. This doesn’t necessarily mean that those visits would convert into customers.