Every rule starts with an IF-statement. With this IF-statement you can select a certain part of your products based on whether these products apply to a certain condition. If you want to select all products, you can leave the IF-statement to "all". You can use this functionality as a filter: for example, as a quick check to see if a certain product is configured correctly. After using the If part of the rule to select products, hit 'Save rule' and you will see the statistics on the right hand side change. Click the 'Selected products' button on the right hand of your screen and you will see all products that are selected (and therefore affected) by this rule.
A common use for filtering is checking whether there are any products that failed to get affected by earlier rules (like a category field being empty). Another common use is adding a rule that selects all products and does nothing at the bottom of the rule chain: using the preview, you get a more clear overview of the final product data than the preview may give you.
See this page for common filters.