From Halopedia, the Halo wiki
Flag juggling is the intentional act of dropping the flag and wielding it again. It is performed by first jumping as you pick up the flag and then dropping it before you land and then repeating that over and over again thus allowing you to circumvent the restricted player speed for flag carriers.
Flag dropping is a common tactic performed in the Capture the Flag gametype when the flag carrier has no access to a vehicle; since carrying the flag reduces the player's speed, constantly dropping it ahead and grabbing it is a precious time-saver, especially when time is running out. Flag dropping is also used as a form of match-time celebration. When someone steals a flag, they may drop and steal it constantly while on their way back to their base. This leads to the narrator repeating "flag dropped" and "flag stolen", to the annoyance of the other team.
The tactic was effectively killed in Halo: Reach due to a new gameplay mechanic that only lets players build up normal running speed again after one second of dropping an objective item, and in Halo 4 which prevents the flag carrier from dropping the flag. Halo 5: Guardians however, allows players to juggle the flag.