Prisms are cheap tools that allow you to create artistic effects on your images without the need for a ton of gear.
In photography, there are certainly many niches and schools on thought on how to capture a given scene. You can have a more documentary approach, where you look to capture things more or less as they are. But you can also have a more artistic and creative approach, looking to use the scene in front of you, molding it with light and other tools to fit your creative vision.
One such tool that has gained popularity over the last several years has been prisms. Their use has become popular among wedding and portrait photographers for their ability to add some creative flare to their images without adding a ton of bulk to their kit. Just throw the prism in your bag and you are good to go. Photographer Mathieu Stern has created a couple of short videos featuring prisms.
These videos are quick views but offer a great demonstration of some of the effects that one can create when utilizing a prism. It is not at all complex or difficult either, it is literally just holding the piece of glass in front of your lens and moving it around until you get the desired effect in your image. This is made easier by shooting with live view or through an electronic viewfinder on a mirrorless camera like the Fujifilm X-T2 or Canon EOS-M5.
Prisms are fairly cheap too, so who not run out and pick one up? There is no doubt after a little bit of playing, you may find yourself frustrated with your results, but just keep with it, keep moving the prism around your lens and sooner rather than later, you will start to see how to get the results you are looking for and all that frustration will fade away.