When designing a Web page, Web developers have the chance to add what are called keywords to the Web page source code to help search engines such as Google to drive users to your Web page. There is always a debate on how many keywords to include, because if you have 5 keywords, search engines typically view them as more important than if you had 100.
Complicating things is that Google and most other search engines also include things like "how many other sites have a link to your Web site" and other such things to rank your Web site.
There are some things you don't have a ton of control over, but you can choose effective keywords that help drive visitors to your site. One of the ways to do that is to use a keyword suggestion tool. For example, the free site listed below will show you related keywords you can use that might get you better placement on search engines.
Again, there are many sites that do this, the one below is an example. However, it can tell you words related to your keywords that you can use to attract more customers.
Keep in mind that this is also a technique less reputable Web sites can use. For example, let's say you have a Web site about the Sopranos TV show. The first thing that shows up on the list above when you look for sopranos is not "Tony Soprano" or "The Sopranos", but "Isabella Soprano" - an American pornographic actress, according to Wikipedia.
If you were really looking to drive people to your Sopranos TV site, you could throw in a reference to the actress and when people Google that actress, they may come across that site. Who knows, in the future, perhaps people looking for this porn star will end up here by accident!
I'd recommend using this for the purpose it was intended - to find popular keywords to help drive traffic to your Web site.