HVDC means High Voltage Direct Current and it enables controllable exchange of Active Electrical Power between two stations since the two stations in case of their being connected over HVDC link. As any electrical engineer would know, flow of power between two ends of a transmission line would comprise two components one of which would be Active Power measured in Watts (W) or kilo Watts (kW) or Mega Watts (MW) etc, and the other component would be Reactive Power measured in Volt Amps Reactive (VAR) or kilo Volt Amps Reactive (kVAR) or Mega Volt Amps Reactive (MVAR) etc. and due to this the transmission line concerned would have to also transmit the VAR/kVAR/MVAR component, whereas needed at the receiving end would be only the W/kW/MW. Thus, the transmission line would be unnecessarily loaded by undesired component - thus the capacity of transmission line would be only partially utilizable. Also, AC transmission lines have Reactance (Inductive and Capacitive) which are directly proportional to line length. Due to this a severe limitation is imposed on transmittable power on it. Further, ordinarily, it is not possible to precisely control the quantum of power flow over AC transmission lines.
DC transmission lines do no suffer from any of these limitation because in DC the product of Voltage and Current is always Active Power (thus entire current flowing into the transmission line pertains only to useful active power) and HVDC transmission makes it possible not only to precisely control the quantum of power flow but also its direction. For example power flow can be from a point A to point B during day time and from point B to point A during night time.
For achieving HVDC Static Rectifier/Inverter Equipment is installed at both ends of HVDC transmission line. At any given time the Static Rectifier/Inverter Equipment at one end acts to rectify the AC power source available at one end to convert it to DC and that at the other end acts to invert DC to AC for further distribution. Which end would act as Rectifier and which as Inverter is changeable as desired.
The reason that HVDC transmission is used when transmission line length is high simply because otherwise power would not flow with conventional 3-ph AC transmission due to inductance of the line and also because Rectifier/Inverter Station are required at line ends, cost of which does not justify the investment for short lines. However, there is no technical limitation of using HVDC Transmission even for a short distance. In case two different power supplies are to be coupled for power exchange and one of them is at a different frequency than the other (for example one power source works on 50 Hz and the other at 60 Hz - like in Japan, where some part of the country has 50 Hz power system and the rest has 60 Hz) only HVDC can be used even though distance may be lesser than one kilometer.