What is the meaning of the letters on the side(s) of WWII British aircraft?
Spitfires had a few different markings, but primarily there were two. One, written in smaller, often black, text was the aircraft serial number and stayed with it for its entire life. The other, usually either side of the roundel was the squadron and airrcraft id:


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XT was 603 Squadron. So if a Pilot of that squadron was told to go and "Fly Delta" he would know to go to that specific aircraft. Aircraft would have been lettered in sequence, Alpha, Bravo, Charlie etc.

There were some exceptions; to wit


  ...Thus the famous Douglas Bader was allowed to have the letters "D-B" painted on the side of his Spitfire VA, because he commanded the Wing flying from Tangmere Aerodrome. Bob Stanford-Tuck, commander of the Biggin Hill Wing had a Spitfire with "RS-T" on it.
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Hurricanes followed the same standard