"La Roja" means literally 'the Red'.
"La Roja" is the common term to refer to the Spanish National Football Team.
This term became popular after an interview with the former Spanish coach Luis Aragones in 2008, in which he said that team players should be proud to wear "the Red", in reference to the red t-shirt.
It was in the same year 2008 when the Golden Age of the Spanish Football started, becoming the European Champions in two ocassions (2008 and 2012) and World Cup Champions (2010).
Before that the Spanish Team was also known as "la Furia Roja" (the Red Fury").
Another team also popular as "La Roja" is the national squad of Chile.