An exclusion is a filter that hides rows from consideration entirely in any analyses or queries involving the dataset (i.e., the dataset behaves as if these rows do not exist):
To create an exclusion, open dataset properties and then select Exclusions (you must be the active editor to do this). This opens an interface similar to the Filter Builder. You can then create your exclusion in a similar manner to building a filter. Note that a dataset can only have a single exclusion, but the exclusion can include multiple variables.