Piece of photographic equipment used when making a print from some form of transparent media such as a colour or black and white negative or a slide. It's basically a microscope in reverse - you put the source material in the head, and a light source and lens group projects the image onto the destination paper.
Enlargers typically have a variety of colour filters to permit colour compensation, and may have an exposure timer built in. Separate focus and enlargement controls allow the user to magnify as appropriate for the print size.
Exact focus can be checked with a grain magnifier which allows the user to view the image on the destination plate at a high enough magnification to see the grain on the source film.