The Key Metrics tool is a block that can be enabled on the Dashboard. This enables you to view a set of key statistics across all modules, at a glance. For example, the following screenshot gives me an easy way of seeing all Companies assigned to me, all Companies set as Prospect, my activities for the day, and how many open tickets there are on the Helpdesk.
Clicking on any of those headers will then take you to the relevant view, e.g. My Companies:
To add a Key Metric to the list, you simply need to tick the "List In Metrics" box when creating or editing a custom view (see this FAQ on how to create a Custom View)
Please note that Key Metrics are set at company-wide level, so any views set to "List In Metrics" will be visible to all users that have the Key Metrics block enabled on their Home page. For details on how to customise your Home page please see this FAQ.