What is a sample covariance?
The sample covariance is a measurement of how greatly variables differ from each other within a sample.
Covariance tells you how two variables are related to each other on a linear scale. It tells you how strongly correlated your X is to your Y. For example, if your covariance is greater than zero, this means your Y increases as your X increases.
A sample in statistics is just a subset of a larger population or group. For example, you can take a sample of one elementary school in the country rather than gather data from every elementary school in the country.
Thus, sample covariance is simply the covariance found within a sample.
The formula for sample covariance can be found here.