In a sentence; it is a well organized set of information made easily accessible to another computer to crawl (the bot) and index ( the search engine ).
Search Engine Friendly means What Humans Don’t See
It is not necessary to have a visually attractive or interesting website in order to be search engine friendly. “Visually attractive” or “interesting” are things humans want. Search engines seek well organized information; a theme, explicit descriptions of a page, and explicit labels (for images). These are things humans don’t see or even care about. Here’s some advice from Google
What is not Search Engine Friendly
90% or more of “pretty” websites are not optimized for search engines (the web designer will always say it is) (and how can the client tell?). In contrast, you will never hear a real SEO claim they build pretty websites – it’s not their talent and good SEO consumers all their bandwidth. Web Designers concentrate on the visual. Do they also have the ability to concentrate on the unseen SEO as well? (I like and use webs designers, but…)
Then there are the website owners, they usually make a mess of a perfectly organized website; they move pages around, change menu navigation which often changes URLs; add this thing; and add this other thing; and soon all these “things” create obstacles for search engines. Over time the web site becomes an unintelligible and very search engine unfriendly mess.
Many website owners and advertising agencies try to do SEO themselves because it seems easy and obvious ( read a book – search online ). Hiring an SEO specialist is an unnecessary expense. We would add to the list that – hiring an SEO specialist seems unnecessary because there is a belief that “everyone can do SEO”. This self-perpetuating environment in which every website designer seems to do SEO, leads you also to ask, Why can’t I do it myself?
“A proficiency at billiards is a sign of a misspent youth” Mark Twain.
When the task is to turn a human website into one that also speaks to, and serves the needs of search engines, hire those who understand how search engines work and can take the time to do it right. Sprinkling a few keywords around pages and posts is not SEO and will not create a Search Engine Friendly Website.