Hello lillian,
Welcome to the board.
If I understand your question, contrast refers to the process of injecting the patient with a solution that acts like a "dye" while they are undergoing an MRI. The purpose is to make it easier for the radiology to be able to see certain things more clearly. Sometimes you have an MRI without contrast, and other times the doctor will want one "with contrast."
At least when I've had it done, you're in the tube for awhile without the dye. Then they pull you out, inject the dye into an IV line that was previously put in, and push you back into the tube for some more imaging.
Hope this answers your question.