The robots.txt file lives in the root of your shop like https://stockphototees.com/robots.txt. It directs search engine robots (the crawlers they send out to index the web) how they should behave on your site: the URLs they are allowed and not allowed to visit.
On Shopify the robots.txt is automatically generated. It has been well written and tested on thousands of popular shops that perform well in search engines. So you can rest assured that Shopify is not blocking anything that shouldn't be blocked.
Blocking is indeed a good thing if done correctly. For example, you don't want Google to index password-protected pages, account pages and pages that have no value to searchers.
If you un-block pages like these you will pollute search results for your shop with confusing and useless pages if the customer lands on them. Say, indexing the cart page has no value because someone searching for your products has nothing in their cart yet and landing there will show them an empty cart! These pages can also dilute the chance other pages on your shop will be shown.
Therefore in almost all cases you shouldn't edit your robots.txt because it is already set up well on Shopify.
if you think you do need to edit robots.txt, email us on firstname.lastname@example.org and we'll be happy to help.