Galactus is one of the most powerful and oldest entities in all the Marvel cosmos, and for good reason. Born from the big bang, the Devourer of Worlds must go from planet to planet consuming their energy to survive, and for this he requires what's known as a 'Herald' - a partner imbued with the Power Cosmic who must find worlds for their master to, well, devour.
The classic and most iconic Herald of Galactus was of course Norrin Radd, dubbed the Silver Surfer by his master, but there've been just over a dozen other Heralds employed by Galactus before and long after the Surfer broke free of his control. It's a prestigious title to hold, after all, and with it comes an immense source of power, as well as a unique story where they either abide by their master's wishes, or conspire against him.
Bargains are made, sacrifices are common place, and even the occasional reunion has come about either because Galactus is in need of help, or because he threatens to do something diabolical.
It's partially why the Heralds are all such fascinating characters, and while some are obviously better than others, few characters can claim to be as cool, or even as storied, as they.