What does alkalinity mean?

The alkalinity (TA) indicates how much acid a medium (soil, rock, water) can absorb and "bind" without changing the pH value. 
The respective degree of alkalinity depends on the amount of alkaline-acting ions contained. The alkalinity of the spa water is considered to protect the pH against unexpected fluctuations.
The optimal value is between 80-160 mg/l. The consequences when the alkalinity falls below 80mg/l are considerable fluctuations in pH to the consequence.
This may cause poor water conditions, which could consequently attack the spa itself.

Here you can find more detailed information about alkalinity and its application.