A contact printer (or proofer) is a kind of easel that enables you to make a sheet of thumbnail prints from a roll of film that has been cut into strips, usually containing six images per strip for 35mm film and three or four images per strip for medium-format negatives. The value of a contact printer is that once you have a contact sheet, it becomes easier to evaluate which negative(s) to blow up into larger prints. Using a contact printer is simple. Place a piece of photo paper on the proofer's base pad. Then, take your negatives and lay them out in order in rows on the paper. Close the glass cover. Take the proofer and put it under your enlarging lens. Once this is done, make an exposure. Then, develop the paper. You now have a piece of paper with lots of small 1:1 image-frame-sized pictureson it.
Some contact printers have grooves in them to hold the negatives in line. Good ones will have a sheet of glass that closes over the negatives and holds them flat against the paper. Some of them leave spaces for the frame numbers to print through. Some are slip-in style where the negatives are slid in from one side and the paper slipped in underneath. Some are like a picture frame with pressure clips on the separate back plate that lock in and press the paper and negatives quite flat against the glass. Choose the style you feel most comfortable with. Contact printers are not expensive but are a worthwhile purchase. Some photographers have even made their own versions. All you really need is a pad and a sheet of glass.