Just in case you haven't heard the term "keyword" before, here's a brief explanation.
Any time someone uses a search engine, they type in a word or phrase. This word or phrase is known as a keyword (even though it can be longer than one word.) The search engine goes out and retrieves all the web pages that focus content on that particular keyword or keyword phrase.
The more relevant the web page is to the keyword, the higher the page ranks in the search engine. Each web page contains a place in the HTML coding to insert these keywords. This helps the search engines determine page content. This is a bit of a simplification as search engines use many other criteria to rank pages. However, keyword focused content pages are the most important on-page criteria.
Deliver poor content that is not keyword focused and you will have a difficult time ranking highly in any search engine.
So, the questions becomes this. How does one choose the proper keyword for any particular web page? The answer is quite simple. You research keywords that pertain to your business and choose the ones that have a high demand and low supply.
The Demand of a keyword is a measure of the number of times people have searched for that keyword in a particular period of time. The Supply of a keyword is a measure of how many total web pages focus on that keyword. Keywords with a high demand and low supply will allow your page to rank higher in the search engines and will drive more traffic.
The best way of determining keywords is to use a good keyword research tool. The purpose of these keyword research tools is to allow you to enter a specific keyword related to your business and then see how many times people searched for that keyword (keyword demand) as well as how many web pages are focused on that keyword (keyword supply.)
In addition, a good keyword research tool will come up with new keywords related to the initial one you typed in, giving you ideas for branching out and coming up with dozens of other high quality keywords.