A domain name is the name of your website. For example: mywebsite.com or mywebsite.org
Hosting is a computer that stores data for your domain name. In order to publish content for anyone in the world to view, you need to have hosting with your domain name.
There are many companies where you can purchase your domain name and hosting. You can purchase them at Namecheap.com, Enom.com, Hostgator or Godaddy.
The short answer to what mobile-friendly means, anyone can view your website without having to zoom in to view any content on your website. You can test your website and get a report back in seconds to see if your website is mobile-friendly for free. You can do so by going to the Google Mobile Friendly Test.
Search Engine Optimization, “SEO” is the term that was given when website owners optimize their site using keywords and phrases which briefly explains what their website is about. The search engines such as Google, Bing and Yahoo have bots that explore the internet and reads your content. Once they have read the content on your website they archive it and place it on the many pages of search results when people are searching for content.
“On-Page” or “On Site” SEO is optimizing your website for a particular keyword(s) or phrase(s).
“Off-Page” or “Off Site” is SEO marketing techniques used to improve your rankings in the search engines.
What do you mean by rankings in the search engines?
Major search engines such as Google, Yahoo and Bing display only 10 websites per page. The highest ranking is number one and is the first result you will see. As you scroll down the page the last website you see (that is not an Ad) is ranked number 10. On the next page you will see another 10 results. The results displayed on the second page are ranked 11 through 20. This same process continues on all the pages your search engine provides.
For example, if you see your website appearing as the fifth result on the third page. Your website is ranked number 25.
A website is the collection of web pages. If a website is a book, then a webpage would be the chapter in a book.