Guard is an ability that gives Protect X to the friendly unit on left and right (if available) It works on all types of unit. As it is similar to Protect it might kill adjacent units if the guarder is killed.
There are some differences between Guard and Protect. Firstly, Guard is limited to 0-2 of unit at the same time (0 happens if no one other than guarding unit is in play) but Protect can help many units. Secondly, Guard will swift the applied units, if the adjacent unit is killed and another one is replaced to be the adjacent unit. Thirdly, Guard is always applied regardless of faction or type of the card.
As the Guarded unit can swift, Guard is not limited to a total of Health Plus 4. It rather depends on how many untis are killed or/and dropped below the life line (for example, the unit sustains life when the current health drops to 2 or less, just like protect 2 does) before the Guard unit is killed.
Guard is very useful to boost attack based units like Silverbacks, Wardogs, Abtu-Kahn units.