An exclusion is a filter that hides rows from consideration entirely in any analyses or queries involving the dataset (i.e. the dataset will behave 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 will open an interface similar to Filter Builder. You can create your exclusion in similar manner to builing a filter. Note that a dataset can only have a single exclusion, but the exclusion can include multiple variables.