ground staff radio controll
Delta, ta[ngo], sierra, whiskey, alpha, etc... are all letters in the ICAO phonetic alphabet, which is a system used to speak letters in a way they can be clearly understood over the radio. Just as letters like "D" and "T" have no particular meaning on their own, these letters can be used to mean many different things depending on the context. 

The letters could be part of a callsign (to identify an aircraft, e.g. "november four four charlie kilo" is a particular Cessna Citation), taxiways at an airport ("turn right on taxiway hotel" means turn right onto the taxiway marked "h"), to spell out the name of a location when needed to avoid ambiguity, really anything where spelling out one or more letters is required on the radio. 

You might want to read up on the basics of how aviation radio communications are conducted to help understand the format of typical communications.