Topic targeting allows your ads to be eligible to appear on any pages on the Google Display Network that have content related to your selected topics. As content across the Web changes over time, the pages on which your ads appear can change with it. To display your ads on those pages, simply select one or more topics that you find relevant for your ads.
You can use topic targeting in combination with individual placements -- which are pages or sites you've chosen where you'd like your ad to show -- and keyword targeting -- which lets you choose the keywords that should trigger your ad to show.