Minicomics are a way for making comic books on a small budget. They are usually hand made. Creators usually use photocopy to make the panels fit on the small pages. The term has a bit of a complex history.
You're probably thinking a minicomic is a small comic -- what is so complex about that? Well there is more to the tale. Originally, there was a standard size. Digest Comics were 5.5 inches wide by 8.5 inches tall, while a minicomic was 5.5 inches by 4.25 inches. But, nowadays there is no standard size. Sizes can range from slightly smaller than a "normal" comic to the size of a U.S. stamp. Most of them have unusual sizes and graphic designs. Minicomics are usually photocopied, but in larger amounts, offset printing might be used. Currently, the term "minicomic" emphasizes the handmade aspect, rather than the actual size. So, by loose definition, one could say a single photocopied page could be a minicomic. Also, it important to know that most creators directly sell these comics (as opposed to getting a comicbook store to do it). They can be sold through the mail, ordered on websites or even sold by small bookstores and publishers that sell zines(in this context it means that is a small circulation publication). Usually, minicomics have no editors, giving the creators more freedom. The United Kingdom has a similar concept, but their version is called "British small press comics".
The term minicomic has also been used to refer to small comics given away with toys, such as the Masters of the Universe and two of the Transformers toylines.