Updated: March 29, 2018
SEO stands for Search Engine Optimization and it is the practice of trying to improve how your site does in search engine rankings.
With proper SEO you can begin to see your organic traffic increase.
You should start implementing SEO when you start to blog but you don't need to be hardcore about it.
When I start a blog I just make sure I have titles with decent keywords in them, do a decent permalink, and make sure to include a keyword in a heading.
Beyond that I don't go hard into SEO until my blog is a bit more established and I can start to see what is working.