MouseJack is the term given to a set of vulnerabilities affecting some popular brands of wireless mouse and keyboards. Using this vulnerability, a hacker can listen in on what you type and also know where you're clicking with your mouse. This vulnerability does not affect Bluetooth devices, the list of affected devices can be found on the website of Bastille Networks Inc. who have reported on the issue in more detail.
If you're considering getting an encrypted Keyboard and Mouse combo, check the Secure Devices section of our store for recommendations.
This video gives a good explanation of how easy it is to use, using devices that you can get from any computer store from as little as US$20 and from as far away as 100 meters.