The "GR" and crown stand for George Rex, the King of England in 1911, when it was made. SMLE is of course "Short, Magazine, Lee Enfield," .303 is the caliber, 2.22" is the case length of a .303 case, B.S.A stands for Birmingham Small Arms, who likely made the rifle. "18 1/2 Tons" is likely the pressure level, but I'm not sure.
The arrow is the Broad Arrow, signifying the rifle was property of the British government (an old sign of crown ownership for centuries.) The III I think is a Mark III, which was a modification of the original where they dropped the magazine cut-off, and I think the volley sight on the side of the rifle.