This Excel tutorial explains how to use T.TEST Function to perform Independent Sample T Test and Pairedsamples T Test.
What is Independent T Test
In statistical inference, we are interested to know whether a small sample comes from a population. To inference using sample mean, when the population standard deviation and population mean are known, we can use Z test to interference the population mean from sample mean.
In reality, we do not have data of the whole population. Without the population standard deviation, we use T Test (also known as Student’s t Test) to interference the population mean from sample mean.
There are basically three kinds of T Test in statistics:
One sample T Test  Test whether a target mean value is equal to the population mean 
Independent sample T Test  Test whether the population mean from two individual samples are equal. For two samples, you can also use One Way ANOVA, but ANOVA is not directional. 
Pairedsamples T Test  Test whether the population mean between paired observations are equal 
Excel T.Test Function is used to find the P value of Independent Sample T Test and Pairedsamples T Test.
Syntax of Excel T.TEST Function
T.TEST(array1,array2,tails,type)
Array1  Required. The first data set.  
Array2  Required. The second data set.  
Tails  Required. Specifies the number of distribution tails. If tails = 1, T.TEST uses the onetailed distribution. If tails = 2, T.TEST uses the twotailed distribution.  
Type  Required. The kind of tTest to perform.

Example of Excel T.TEST Function
The below example shows employees’ salary in department A and department B.
The returned value of T.Test Function is a P value. If P value is <0.05, we can conclude the two sets of data have different mean; otherwise the two means are not significantly different.
If you want to understand the meaning of each test, please click the link below.
Outbound References
https://support.office.com/enus/article/TTESTfunctionD4E08EC3C545485F962E276F7CBED055