On the Explore page there are two places where you can find definitions for metrics.
In any table in Mosaic, in any table, the button in the column header is a button you can click to open the definition for the metric in the column.
When clicked, the definition for the column opens.
Click the again to close the definition.
Clicking the button will allow you to add columns of metrics to the Explore page table.
When the Add Metrics interface is opened, you can click on any button to open the definition for that metric.
Click the button to close the definition.