Once you have a Count, Lookup, or Rollup field you can toggle the "Include Filters" option and click on the arrow to define the filters. This allows you to restrict the set of data that is considered for the calculations in the field.

The conditions you're able to specify are identical to the other areas of the product. You're able to specify multiple conditions for any filter. For example, in the screenshot below I'm setting calculation to only consider order records where the status is not "Client Approved" and the Due Date is before Today so I can have a count of all outstanding tasks.