Sitelink extensions add a few extra links (2 - 6) to your ads allowing people to go to different pages beyond the focused one your ad points to. For example, you can run an ad focused on volunteering and pointing to a volunteer sign up page but have a 'Donate' sitelink that someone could click and make a donation if they were so inclined.
This can be a great way to increase click through rate (check out our glossary for definitions) as, particularly with charities, nonprofits, and the Google Ad Grants program, people aren't always searching for what you and we want them to do so we can show an ad based on search but allow for other options they maybe are more interested in but just weren't searching for. Like donations.
It also allows you and us to highlight or feature a campaign, event, or something else time sensitive in an easy way.
Here's one example of how it can show up:
You can read more about sitelinks on Google's site here.