VOR = VHF Omnidirectional Range beacon.
It's principle is based on timing. It's easy to understand if you know how the (VERY) early navaids worked.
Just imagine a lighthouse, with a beam of light that makes one turn in 36 seconds. Each time the beam is heading North, a red strobe on top of the lighthouse flashes. Now if you count the seconds from there, until the beam shines in your direction, and multiply it by 10, then you now your position relative to the lighthouse. For example if you counted 27 seconds, that means that someone standing at the lighthouse can see you when he looks to the West. It is said that your are on radial 270 of that VOR beacon, and your QDR=270. You are in the aircraft and of course you will have to look into the other direction to see the lighthouse, you'll be looking at the East, your QDM=90 (the Q-codes are not very often used anymore though).
A modern VOR works on the same principle, but instead of beams of light, radio transmissions are used, one fixed frequency (the one that you tune into) and a variable frequency that varies according to the direction of transmission.
Note that the info you get from a VOR DOESN'T say anything about the direction you're flying in (the heading of your aircraft). Only the POSITION of the aircraft. But when you're on radial 270 and your heading is 270 then you're flying away from the beacon, it is said that you're outbound and on your HSI, the "FROM" flag will display. But if you're heading is 90, then you're flying dead straigt to the beacon and a "TO" flag will display on your HSI.
Hope this helps.