Used for: setting a field to a certain value.
With the "set to value" rule you can easily add missing information to your feed or change the information that is already in your feed. For example, for a Google Shopping feed you will need to have a gender field, which is set to either "male", "female" or "unisex". You can use the "set to value" rule to easily add this information.
The screenshot below shows you how:
With this rule, the "gender" field will be set to "male" for all of your items that have a product_type containing "men".