Did you see it on the site? Here's an explanation - https://en.wikipedia.org/wiki/Medium-density_fibreboard.
Hi Tricia, this article explains it really well and gives the pros and cons of using MDF too - www.modgsi.com/What-The-Heck-Is-MDF-Anyway_b_24.html
Here's an overview and some more tips on using it:
MDF stands for Medium Density Fiberboard and is basically layers of paper glued together. It can be used to build cabinets or make decorative signs. It is also very economical though heavy! The drawbacks are the use of special fasteners and pre-drilling all holes. Plus, it has to be sealed well because if it gets wet or damp, it will swell. I like to use it to make decorative signs and artwork to hang on the wall. I hope this helps!