This is a relation field, which links to the Tools database. If you're collecting data about this specific metric from multiple sources, you can select them all here.
*If you don't find your tool/data source listed, you can always create a new item in the Tools database.
Here you'll see any objectives that have been linked to a particular metric. Typically, it's best to set the metric while creating an objective.
The totals field is a 'rollup' of the sum of all values within a given metric. Say, for example, the metric was 'Product Sales', but you had 5 or 6 different products on sale. This 'Totals' field will take the values of all objectives with the 'Product Sales' metric attached, then add them to provide a combined total.
Again, the metric is preset with an 'Action' or 'Outcome' tag to help you filter and sort based on 'things in your control' and 'things that happen for your business'.