Search Engine Optimization is the art of making web pages in such a way that any Search Engine can rank it easily while keeping the information of page interesting for users and clients.

Every site page which has to be optimized for keywords needs a detailed testing. This act involves the following three sections.

1. Keywords selection for a particular Subject
2. Description of the webpage content which helps Search Engines for getting information of that page
3. Required amount of a Keyword in a page
 

When people search on web using any Search Engine, they use different words to gain information or data. So each one of those words or the combination of words become important for your website. There are some keywords which are used by more percentage of people while some are used by less people.
Lets consider a example to define this.
If people want to search for Cancer Treatment, there can be following combinations that are used.

cancer treatment 1,338.0 /day
best cancer treatment 638.0 /day 
cheap cancer treatment 415.0 /day 
domestic tips on cancer 175.0 /day 
cancer precaution  74.0 /day

So, its clear that a person who distribute the knowledge about cancer should optimize his page for the other keywords as well. which are listed here. Here cancer is the most important keyword & treatment, best, cheap, domestic & precaution are the Related keywords. The proper amount of related keywords can help a webpage to gain better traffic through search engines.

Once you are ranking for a keyword in Top 10 on Google or some other search engine. The description for the page comes into importance.

Consider the way the internet searcher thinks
Your website is ranking on top for a keyword
Cancer Treatments
But your web page description is like this.
Case 1
This Medical guide provides knowledge of all health related diseases.

Case 2 This Medical guide provide the information on all types of medical diseases, specially all type of cancer, precaution on cancer or to find cheap cancer treatments.

In Case 1, the crawler is not much impressed as he was looking for Health related diseases and he found you a "general website".
In Case 2, the crawler will click more to get the knowledge of cancer treatment and to know the types of cancer.

That is the main importance of good description for pages of your website can bring to you. The reason is simple, the person will only come to your website if he is impressed with description of your page on Search Engine.