The Key Transactions feature allows you to analyze the performance of specific web transactions at a glance. Let's say you want to monitor the metrics of certain transactions, like those made to a payment gateway, login requests, or the action of fetching customer info from the database. In the normal view, you have to dig these metrics out from the bucket of other transactions. 

Instead, you can mark these transactions as Key Transactions in APM Insight, and view them separately. This helps you to identify and resolve specific transaction issues faster. 

To mark a key transaction, just hover over the hamburger icon of the concerned transaction, and mark it as a Key Transaction. 

Learn more about key transactions. 

Hope you found this useful.