Double hyphen / dash in SQL injection. What are they used for?

Question

I am currently studying how SQL injections work. Many training sites have examples, such as

SELECT fieldlist FROM table WHERE field = 'x' AND email IS NULL; --'; 

In the "field" field, the content that will be checked comes from a text field or something similar from the website. In this case, user input x' AND email IS NULL; -- x' AND email IS NULL; -- What is the use of -- ??

I saw several similar examples, but no explanation. Am I missing something fundamental?

Answer 1

Oh never -- used to comment on the remainder of the request ...