You can add a “+”modifier in front of the words in a broad match keyword 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
Broad match modifiers let you target searches that include at least one of your keywords. This can help increase how relevant your traffic is to your ads, and improve your clickthrough (CTR) and conversion rates.
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