MCAR is an acronym for Missing Completely at Random
MNAR means Missing not at Random
When you have data missing you may try to check if this missing data is random or not.
Usually this is done by checking the average of other variables for the cases there is data missing in one variable or no data is missing.
A t test for the difference of the means can tell you if the missing data shows the same behaviour.
A difference is enough to tell that they are not equal but for say that the missing data is random it must be checked against all other variables.