Broad Match Modified:

With modified broad match, you put a plus sign (+) in front of one or more words in a broad match keyword. The words that are preceded by a (+) sign must appear in the user’s keyword phrase exactly or as a close variation.

Let’s take our hat example from the Broad Match match type:

…when you add “hats” as a broad match keyword, you tell Google Ads to try to show your ad for searches containing that term or a similar one. Your ad might show for searches on “hat,” “sun hats,” “winter accessories”, and “sombreros.” Broad match keywords help you reach the widest audience.

If we wanted to be sure our ad only showed on ads where people were searching for “red hats” then we could use the broad match modified +red +hats.  This would ensure that the words red and hats (or a close variation) would be required in the search before our ad showed.