The Galacticos were, and are, top football players signed to Real Madrid. The word was first used in this context around 2007. The then manager of Real Madrid, Florentino Perez, introduced a policy of recruiting star players (galacticos means galactics or stars). During the 7 years that he was in charge, the team signed up some world famous players at very high prices. These were Zinedine Zidane, Ronaldo, Luis Figo, and of course David Beckham (Beckham was actually the cheapest at £25 million!) Some people include Michael Owen in this list too.
Apart from these original names, it's not always clear which players should be considered galacticos and which are just 'normal' players. Real Madrid and other clubs have of course continued to recruit the best talent from around the world, but does this mean that all their players are galacticos?
The word is also sometimes used outside football, to describe an elite or star group of people.