Health items are used for restoring health or going above health limit. There are two types of health items in Quake Champions, Small Health and Mega Health. Those items can be found on any arena.

SMALL HEALTH
Small Health is an item pickup specific to Quake Champions and was not present in past Quake iterations. It still has the same basic function, replenishing health up to maximum health, but it achieves it in a completely different manner. While old health items restored a fixed amount of health (5, 25 or 50) the new health restores based on health missing from health containers. The range of health that will be restored is from 13-38.

EXAMPLE
If a health container (box on HUD, 25hp each) is more than half damaged (12hp), Small Health will restore that container thus healing 13 health.

If a health container is half damaged and there is additional health container available above it, it repairs the current container along with the next one, thus restoring 12 points of current container + 25 points from the container above, making it 37 health restored in total.

MEGA HEALTH
Mega Health item was present in previous Quake titles, but as well as Small Health, it works completely different now. As opposed to before when a Mega Health pickup would boost a player for 100 health, now it boosts him for maximum health + 50.

EXAMPLE
The player is playing as Nyx. This champion has maximum health of 100, therefore a Mega Health item pickup would grant Nyx additional 150 health. Both Nyx on 100 health and Nyx on 1 health would get restored to her maximum health with decay limit (overMax), which is 150.

Source: PAX gameplay videos