A dog playpen or exercise pen are a six to eight hinged panels that form a closed ring that you can put your dog or other types of pets in to get some semi-supervised time either indoors or outside. Dog playpens come in different sizes and are constructed out of steel wire, similar to the wire dog crates, plastic or newer popup variety that are similar to soft dog crates.
Advantages of owning and using an exercise pen is they will, as does a dog crate, help speed up and simplify house training. You can always have your pup in training close to you due to the portability of the exercise pen. You can put your pup or dog in the pen while others are visiting giving a good area for confinement other than the crate. The pen provides a safe enclosed area for a puppy or dog outside, instead of running all around the back yard. Pens are light weight and easy to put up so they are ideal for confined areas while traveling. The area in a pen could be a good place for a recovering dog that has to be confined while getting over surgery or an illness.
Disadvantages of using an exercise dog pen is caused by the portability. The pens are light weight and portable therefore not readily suited for a large breed pup. Most are too small for larger dogs in which case a dog kennel may be better suited. There are no anchors for these types of exercise pens. I imagine you could get some u-shaped garden steaks to anchor this down well enough to keep it from moving.
If you have a reserved puppy or a small breed pup or dog a dog playpen may be a very good alternative for you to be able to let your dog get a little exercise while you get some things done around the house. These exercise pens work very well for other types of small pets as well such as cats, rabbits, guinea pigs, etc. Some of the newer soft sided pens come with tops to give a completely enclosed area.