I'm playing in column 5 to block my opponent's horizontal threat. They currently have two X pieces in a row at positions (5,3) and (5,4). If they were to play in column 5 next, they would have three in a row and be one move away from winning horizontally. By placing my O in column 5, I prevent this immediate threat. Additionally, this move gives me potential to build my own opportunities on this side of the board without giving my opponent any immediate advantage.