A key event is an event that measures an action that's particularly important to the success of your business. When someone triggers the event by performing the action, the key event is recorded in Google Analytics and surfaced in your Google Analytics reports.
Any event you collect can become a key event. To measure a key event, create or identify an event that measures the action and then mark the event as a key event. After you mark the event as a key event, you can see how many users perform the action and evaluate marketing performance across all channels that lead users to perform the action.
The following flow shows how to mark an event as a key event. In short, if an event is important to your business's success, you can mark the event as a key event in Analytics.
Event → Key Event
Example
It's important to know when a user scrolls to 90% of a lead-generation page on your website. When a user scrolls to 90% of the page, you know that the user has read through the page and viewed the sign-up form. In this example, you can identify the 'scroll' event on the Key events page in Admin and then mark the event as a key event, providing you with more enhanced behavioral reporting.