VOR's, or Very High Frequency Omnidirectional Ranges, and DME, or Distance Measuring Equipment, is a technology used to provide accurate navigation assistance across 360 degrees around a VOR station. 

Unlike NDB's, VOR's require a line of sight in order to function - due to the 'very high frequency' radio waves used, they are unable to bounce off the atmosphere as would otherwise occur with lower frequencies. 

VOR's function by transmitting two signals at the same time - the first signal is constant in all directions, and is used as a reference for the VOR receiver. The second signal is intermittent, rotating through 360 degrees around the VOR. Using the difference between these two signals, the receiver is able to determine the direction from the station. 

Additionally, to assist with following a particular 'track' towards a station, the receiver is capable of determining the degree by which the aircraft deviates from a selected course, displaying that in the middle of the VOR dial.

A VOR dial, labelled.
https://www.thaitechnics.com/nav/tg6/vor_indicator.jpg