It means "to make something clear" or "to clarify something". you use it while discussing something - for example - "let's get this straight" when you want to make sure that you and your partner understand something correctly and your points of view are similar

You rather say "get something straight" ,which means to understand something clearly or express it frankly. Examples: "You better get this straight, you are not supposed to fool around with John's wife." "Let me get this straight,I won't be able to contact her ever again!"