Significance / Plan behind the move White's last move was Ra2. I was to curious 
to know the plan behind or any strategical / tactical significance 
behind that move.
Ra2 moves the rook off the diagonal of the g7 bishop thereby preempting any tactical tricks that might occur later. It is better than Rb1 since then Qa5 would embarrass the bishop on a3. 

It also leaves b1 free for the queen later when white wants to exert more pressure down the h1-a8 diagonal of his g2 bishop against b7 and c6. Black has a passed a pawn and white must generate pressure in this area in the medium term if the pawn is not to win the game for black in the long term.