For today's installment of (Ad)Word of the Day, we're looking at keyword insertion from the AdWords Glossary:
Keyword insertion is an advanced feature used to dynamically update your ad text with your chosen keywords. You insert a special modification tag into your ad text to enable this feature for your ads.
Keyword insertion is a powerful feature that can save you time and also help make your ad text more relevant. This feature will automatically customize your ad to a user's query, which means your ad is more likely to attract a user's attention. And, since ad text that matches a user's search terms is shown in bold, your ads will stand out even more.

Even with these benefits, keyword insertion shouldn't be used all the time. We recommend reading this article and going through this tutorial before you start using keyword insertion.