Alkalinity, specifically, is the buffering capacity of the tank, or basically the ability of the water to withstand changes in pH (basically a drop in pH due to the addition of an acid - carbonic acid, produced from the respiration of organisms - CO2 + H2O = C2CO3 or carbonic acid).
A low alkalinity (and there are a number of different units used - meq/l, ppm, dKh) means that there is a greater risk of fluctuations in pH. pH is generally lowest in the morning after the lights have been off many hours, as only repiration is occuring in the tank, leading to a build of of CO2 (and hence carbonic acid). During the day, photosynthesis also occurs, using some CO2 and resulting in a generally slightly higher pH after the lights have been on for several hours.
The amount of CO2 in the system is a major factor in the determination of pH problems in a tank, and why circulation is always a critical factor.
Low alkalinity and the resulting pH fluctuations can be fatal in some circumstances. "Messing" with alkalinity and pH without also monitoring calcium levels can also have undesired consequences.
There are a number of posts on this topic (alkalinity, calcium and pH). I recommend searching for them.
To Start:
https://www.saltwaterfish.com/vb/show...hreadid=137590