What is the practical meaning of the variance, covariance, mean value?
Mean of a signal can be practically visualized as the dc average value present in the signal (for a complete sinusoidal period), for e.g



Variance of a signal is the difference between the normalized squared sum of instantaneous values with the mean value. In other words it provides you with the deviation of the signal from its mean value. It gives you the spread of your signal's data set.

Co-variance of a signal consists of unnormalized relation with the signal correlated with itself and also with other signals. It provides you the degree of similarity between the signals of interest.

https://en.wikipedia.org/wiki/Variance

http://www.dspguide.com/ch2/2.htm

https://en.wikipedia.org/wiki/Covariance

EDIT : FOR COMPLETE INSIGHT PLEASE REFER ALSO TO @Dan Boschen's ANSWER.