Cronbach's Alpha is a test statistic to measure internal consistency and reliability. I never covered it in any of my statistics classes as the datasets I used did not suit this as a test. It seems to be very suitable for data like survey responses and to determine if questions on the survey correlate with each other strongly enough so that you can conclude they all measure the same thing (the subject of the survey). If, for example, you were conducting a survey on attitudes to Medical Insurance, you would want to make sure that your survey questions measure the same thing (attitude). You would expect respondents who give a high rating to the statement "I like the quality of my Medical Insurance", to also rate the statement "My Medical Insurance is good value for money" highly.
Excel does not have an option to calculate Cronbach's Alpha, so a workaround is necessary. This is a very easy test to do in SPSS - here's how to do both: