Thanks for your question.
The abbreviation, "cf", means "compare".
You have just cited a chapter and verse but no book reference. Usually a writer will give an abbreviation of the book too. For example, if I were writing about the book of Matthew and in a comment on Matthew 1 verse 1, I wanted you to look at the meaning behind the reference to Abraham and David, I might say something like cf Gen. 12:1-4; 2 Sam. 7:12-14.
The only time I wouldn't mention the book is if I were asking you to make a comparsion within the same book. So, if you were reading something about the book of Isaiah and the writer wanted you to go and compare Isaiah 42:1-9, he would just put cf 42:1-9. In other words, you are already in Isaiah so he is sending you to chapter 42 verses 1-9 of that book.