From the Numbers built-in help:
The TTEST function returns the probability associated with a student’s t-test, based on the t-distribution function.
You can download the free Users Guide and Function reference here which may help:
The TTEST() function appears to return the p-value of the specified t-test.
Numbers function guide is rather useless in finding precise definition of each mathematical function. If only it gives the exact formula representation of functions rather than vague and inaccurate commentaries.
statistical significance (p value)?