You can filter or exclude products for various reasons. One example being products out of stock. First you have to create a rule and you have to make sure the products you want the rule to be effective for meet the 'if' criteria.
If you want to exclude products: 'Then' take 'all fields' and 'exclude'. In the example below all products with 'Red' in the field 'color' will be excluded.
It is also possible to apply other actions to the selected products, see this page for common rules.