Pushing and Open Pushing

This is a new term I am coming across and was hoping someone could explain it better, and when to do it. Also, I am currently reading Lee Jones third edition of Winning Low Limit Hold Em (after having read TOP of ITH) and he mentions raising medium sized pairs for expected value in late position, and on the big blind, when there are a few callers in front of you. Is this applicable to NL SNG's and could someone explain the concept a little better

Trev