Some comic books are only planned for a limited run and are called a limited series or mini-series. Then there are those ideas that were never meant to go for more than one comic book, and these are the one-shots. A one-shot is a comic that is designed to only have one issue.
For one reason or another, these one-shots are brought to life in an attempt to make a splash, as part of a promotion, or maybe to make a quick buck. They are also often used in tie-ins to existing properties like movies, video games, or television shows. Sometimes you will find a one shot inside a toy package as part of the promotional material. There have been some one-shots that have become a serialized comic book when the buzz and sales were strong enough.
The concept of the one-shot was stretched a bit with Marvel's What If? series. Each What If? comic had their own theme that asked a what if question. The stories were almost all one-shots that wrapped up in a single issue. Some of the titles were, "What if Spider-Man joined the Fantastic Four?" and "What if Storm of the X-Men had remained a thief?" It is ironic that some of these what if scenarios actually became storylines in the normal Marvel Universe.
At any rate, one-shots are a common part of the world of comic books and can be a great read. Some are more of a throw away and will often not be worth much if anything.