Explanation:

What is the Terminal High Altitude Area Defense System (Thaad)?
? Shoots down short and medium-range ballistic missiles in the terminal phase of their flight
? Uses hit-to-kill technology – where kinetic energy destroys the incoming warhead
? Has a range of 200km and can reach an altitude of 150km
? US has previously deployed it in Guam and Hawaii as a measure against potential attacks from North Korea

China’s and Russia’s concerns regarding its deployment in South Korea: Despite having a range only of 200km, the system has powerful X-band radars that can “reach” far; this could be used to spot Chinese/Russian missile launches and thereby assist US defensive systems, potentially affecting China’s/Russia’s deterrent capability.

http://indianexpress.com/article/what-is/what-is-thaad-everything-you-need-to-know-about-the-us-anti-missile-system-4630087/