Meta Tags
Whenever someone searches for a keyword that you're site ranks for they will see the meta tag in the search engine results.
Where Can I Change My Title & Descriptions?
From your Shop's admin area, head to Settings > SEO/Tracking.
The first panel can be used to quickly showcase what your shop sells and give potential visitors a reason to visit your shop.
Note the character limit for both the title and description. Best practice for title's is to put a keyword you want to rank for and separate any words with a bar like this: |. For the description best practice is to keep the description between 120 and 150 characters.
A cautionary note: don't try to put every keyword you want to rank for in the title and description. None of the search engines appreciate this "keyword stuffing", in fact this will hamper your SEO efforts with some search engines.
Robots dot Text
Moz, the unofficial authority of Search Engine Optimisation, define Robots.txt files as "a text file webmasters create to instruct web robots (typically search engine robots) how to crawl pages on their website."
In essence, the text file tells any bots that are crawling the pages on your site where they can and cannot look/record. This could be bots from Google, Bing, Yahoo, or other perhaps more nefarious sources.
As these files can severely effect the SEO of your site, it is advised that only advanced users add, adjust or manage these on your shop.