MuP - MirrorUP.
Useful in BULB mode. otherwise it's just a delay of the shutter release.
It is a feature on the top left dial that allows you upon first pressing the release (shutter button either on the camera or a remote release) to Raise the Mirror. Then the Next press of the release will activate the Shutter. It's up to you to press the shutter a second time. So you could wait 1 second, 3 seconds, 10 seconds, 1 minute .. etc.
Then the 3 time you press it will close the Shutter for any long exposure. So you can manually take a long exposure, or use a timer, etc.
The lower cameras I believe only have a 3 second wait which I've found in tests to not be long enough to prevent the MirrorSlap from vibrating the entire tripod setup/camera. Depending upon tripod, wind, not moving as even on a cement pad I've notice star movement from walking on the cement pad. So I've always preferred the MuP feature in Nikons. Some Canon DSLRs have electronic shutters (like in Mirrorless).
You can use MuP for anything. Astro work, landscapes, architectural, any time you want to control the time from mirrorslap, to shutter release. I use it for many things including when I test a camera/lens' combo image IQ. There's really nothing to "show" you in relation to MuP.
Mirror Lockup (MUP) on the Nikon D7100 - dummies