A literary agency asked me to submit to them, what does it mean to talk about your credentials?

"Please send us a query letter of no more than two pages, which includes your credentials, an explanation of what makes your book unique and special, and a synopsis." What is a query letter? How am I going to include my credentials when I do not really have none ( I just turned 18 in April) all I have is journalism in middle school and high school. That is about it. I know what makes my book special and the synopsis.