Mouseover Heal Macro
WoW Classic Era Shaman Macro
While this type of macro can be used with crowd control or damage spells as well, it is especially useful while healing, as it allows you to instantly heal someone in your raid frames without needing to select them first. That will improve your reaction times, which are critical for healers, and allow you to keep an enemy target selected, to track their casts in case a quick interrupt is required.
#showtooltip Chain Heal /cast [@mouseover] Chain Heal