It's a term used in rps that is used when someone does an action/actions that are impossible to defend against. It is also used when someone describes how the other person is affected by it, thus leaving the player unable to rp his/her own character.
For fighting, it's being able to dodge everything and always hit your opponent. For other things, like using something that takes over someone's mind, it's saying that you take over their mind and do this or that to them without allowing them to figure out if they can defend against it.
Being a God Modder is a sign of douch baggedry and makes it to where other player's do not want to rp with you.
Fighting: person A dodged person B sword attack, and then dodged the lightning bolt from the sky that person B cast. Person A then Hit's person B, sending person B flying, then hit's person B with a huge fire bolt, thus burning person B.
Person A wins the fight and harms person B because person A made it impossible to hit him, then he made it to where B was affected by everything.
Mental: person A cast's a spell on person B, now person B is under person A's control. Person A then tells person B to go commit suicide, person B is now dead.
Person A get's rid of person B do to god modding because they forced the other person to be affected by it and say how B reacted without letting B do anything.