First this is what robots.txt is:

he robots.txt file is used to instruct search engine robots about what pages on your website should be crawled and consequently indexed. Most websites have files and folders that are not relevant for search engines (like images or admin files) therefore creating a robots.txt file can actually improve your website indexation.

If you don’t know how to create this file for your WordPress page this link can help you with it.

Here is a link to full post with Robots.txt example file : Creating A WordPress Robots.txt To Improve SEO.