From Halopedia, the Halo wiki
Stamina is a feature in Halo 3: ODST. When the player starts to take damage, their stamina will start to decrease. When it is getting low, it starts to flash red until the player's stamina is gone, in which case, their health will start to be affected and decrease as well. However, stamina can be recovered if the player takes cover and stops taking any damage.