You can add a “+” modifier in front of the words in a broad match keywords to:
- specify that someone’s search must include certain words or their close variations
- indicate that this keyword should be dynamically inserted in your ad text
- specify that certain words and their close variants be prioritized
- override a negative keyword with a positive one
By adding a modifier, your ads can only show when someone’s search contains those modified terms, or close variations of the modified terms, in any order. The modifier won’t work with phrase match or exact match keywords.
Unlike broad match keywords, modified broad match keywords won’t show your ad for synonyms or related searches. For this reason, it adds an additional level of control. Using broad match modifier is a good choice if you want to increase relevancy even if it means you might get less ad traffic than broad match.
Read more here: https://support.google.com/partners/answer/2497836?hl=en