Subject: What is meant when a person refers to positive and negative energy ?

I'm asking this question because I was given an assignment over my holiday 
break. In class, we just got off the topic of the periodic table of 
elements, and how ionic sizes increase down a group, and how nuclear 
charge increases across a period, etc.. This question is to further our 
education, however, I can't seem to find the answer anywhere. Also, i had 
the question as to what is "shielding" in the periodic table. I know it is 
constant across a period and increases down a group but I don't know what 
it is or why this occurs. Thank you

Re: What is meant when a person refers to positive and negative energy ?