From Halopedia, the Halo wiki
Keepaway (also referred to in-game as Keep Away) is a gametype in the Strategic Options Pack DLC for Halo Wars alongside the Tug of War and Reinforcements modes, and was later included in Halo Wars: Definitive Edition. Keepaway serves as the Halo Wars adaptation of "Capture the Flag". Teams battle it out to capture a free-roaming Forerunner Sentinel to score points before the other team. To win, players must capture three to five sentinels to claim victory.
One minute into a match of Keepaway, a Protector Sentinel marked as the "flag" spawns in a random location the middle of the map. Sometimes the Protector can spawn near an occupied base; once spawned, the Sentinel will periodically move around the map before stopping briefly.
To "take" the flag, a player must move a unit to pick up the flag; the unit will then have to be protected for a set amount of time. While holding the flag, the unit is visible on the map to enemies even when in the fog of war, and is able to fire at enemies like normal. Once the unit has taken posession of the flag, a timer is shown on screen for two minutes; the player must defend the unit until this timer counts down to zero to earn a point. If the unit is killed, the flag resets and the point is not earned. A team requires three points to win the game.
The flag can be picked up by scout, infantry, vehicle air and hero units alike. The erratic movement of the Sentinel can make the flag difficult for slower units such as tanks to reach, though they are capable of taking it if they can catch up.