Sample mean and sample covariance

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Let xij be the ith independently drawn observation (i=1,...,N) on the jth random variable (j=1,...,K). These observations can be arranged into N column vectors, each with K entries, with the K ×1 column vector giving the ith observations of all variables being denoted xi (i=1,...,N).


I have been reading the following link
http://en.wikipedia.org/wiki/Empirical_mean
to learn the Sample mean and sample covariance. But I am failing to understand the above part. Could someone please explain it to me in a easiest way?
 
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When a Wikipedia article is too schematic or abstract ... just do a search using the key words plus tutorial.
Then look over the articles found to find one that is more concrete such as this one:
https://online.stat.psu.edu/~ajw13/stat505/fa06/07_propmean/04_propmean_summary.htm
 

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