On page SEO refers to the code and content on the webpage.
Key or important html tags include:
Making sure that your webpage is laid out properly:
<html> <head> (META TAGS< CSS AND JS SCRIPTS) </head> <body> (WEBSITE CONTENT) </body> </html>
The following are key aspects the go in your <head> tag:
<title>Webpage title - short description</title>
<meta name="description" content="This is the webpage description. It shows up under your webpage title in search results.">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta name=viewport content="width=device-width, initial-scale=1">
View port helps Google identify mobile friendly sites which is a big part of SEO.
Now onto your webpage content, make it highly relevant and including keywords used in the Title and Description. Include image/s if possible and links to credible resources for your sources. Make the webpage responsive and with social share buttons if possible. Make sure that it can be crawled! using robots.txt ensure bots can get data from your website to index with on search engines. Lastly if using images ensure you make use of the alt factor.
<img src="blue-diamond.jpg" alt="Jools Blue Diamond">
Alt is the image description or name of that image and will help with indexing and google image indexing.