The more specific you can be with your keywords, the more relevant your sponsored listings will appear to the users who choose those keywords, and the more likely they'll be to click on your ad. This is where the use of modifiers plays an important role. Many people search on generic terms like "apartment," of course. But a large number of those users are looking for a specific kind of apartment - cheap, luxury, spacious, oceanfront, etc. What are the specific adjectives that apply to your product or service? Add those modifiers to your core keywords, and your list of search terms could increase exponentially.