Search Engine Optimization (or SEO for short) is a method of making a website easier to find via search engines. A lot of this has to do with the way that the content of a website is written. When properly optimized, a website, article or blog can be found easily by a web user searching for relevant words or phrases - without having to look through the literally millions of websites in the net for what they are trying to find.
This is a very basic explanation of what search engine optimization is and of its purpose. But what does all of this mean for you as a writer? Well, it should give you a good idea of how the use of these words and phrases which people enter into search engines when they are looking for something is important to making the content that you write easier to find on the web. These search terms are known as key words and key phrases in the field of writing content for search engine optimization.
Now since you are probably writing this article on the behalf of someone else, how many times should you use this phrase in the article? That all depends on what your client has asked for. You might see search engine optimization articles which use the target keyword just once; in the title and others which use the keyword more than 20 times
Should you use a single keyword in this article or a group of keywords which are relevant to each other? This will again depend on what the client wants. You're not the only one working with the client in many cases; your client may be subcontracting this work to you and have clients of their own who have criteria that they want met with the article. Freelance writers see a lot of this situation; for instance, a set of articles to be written for search engine optimization which come with a detailed set of instructions for how they are to be written. When there are instructions, you should try to follow them as closely as humanly possible.
These instructions tend to be clear and easy enough to follow, with no follow up information needed from the client. However, when your business is writing content for search engine optimization purposes, you will in occasion have a situation where you may need some clarification from a client; in these cases, you should contact them as soon as possible to ensure that there is no delay in delivery and that the article or articles will be ones which your client is pleased with.