what is short covering, short selling. how sensex/nifty climbing/falling ? is it for volume or price traded?

can give me an expert opinion on above
further, if i want to buy xyz share in after market hour hopping that price will jump what should be my strategy if xyz last closing price is 100 (if i expecting sensex will open 150+ next day) and vice versa shorting stradegy….
Repected Jain Saab
Thanks for answering the my quarry pl clarify my these question

further, if i want to BOOK xyz share in “after market hour “hopping that price will jump what should be my strategy when xyz last closing price is 100 (if i expecting sensex will open 150+ next day) and vice versa shorting stradegy….