keywords are words and phrases that determines and identify purpose of your search... http://en.wikipedia.org/wiki/Keyword_(Internet_search)
As stated above, keywords can either be a single word or a combination of words (phrase): ie. "car" or "how to buy a car" The types of keywords you use (and their frequency) will determine the range you will earn per click. It is important that you find nice paying keywords with LOW competition. (Don't go on Google and search for a high-paying keywords list). Thats all there is to it!
If you target is "travel", you should writing some travel article. below is high paying list it may useful for you 500 High Paying Adsense Keywords 300 High Paying Adsense Keywords