Polo shirts are a menswear all time classic which started out as a sportswear for tennis  by French tennis champion Rene Lacoste it has since been adopted by various menswear designers into their collection over the years.
They are a smart/casual wear which can be worn with jeans or chinos paired with sneakers for that smart/casual look. Here I am wearing an NN.07 Polo shirt with a Saint Laurent Jeans and Common Projects Sneakers. 
Shop various designer Polos at MR PORTER