What is the meaning of "expected value of an estimator"? I am studying statistics and i am having trouble understanding some proofs because i don't quite understand what the concept of "expected value of an estimator" means and what is the difference with the value of the esimator itself. 
Say i got a sample and I take the variance v of that sample. That variance v is my estimator. What is the meaning of the expected value of the variance?
Thanks a lot!

The sample that you take is a random sample from your population, so the sample variance $v$ is (at least before you actually take the sample of the population and compute the sample variance) itself a random variable.  If you can figure out the distribution of the sample variance, then you can find its expected value. 
In general, once we have the sample in place, the estimator that we compute is a fixed value that depends on the actual sample that we got.  Until we've taken the sample, it's a random variable that we can analyze in terms of expected value, variance, etc.   
