What exactly is an NFL Touchdown?

I don't watch American Football, used to but not much now-a-days. The question I have is: Why does a player not have to TOUCH DOWN in the end zone in order for the points to count? I don't remember if it's always been this way or not but tonight I was watching the BRONCOS at the FALCONS and on one... show more I don't watch American Football, used to but not much now-a-days.
The question I have is:

Why does a player not have to TOUCH DOWN in the end zone in order for the points to count?
I don't remember if it's always been this way or not but tonight I was watching the BRONCOS
at the FALCONS and on one of the plays, the first score I think, all the player had to do was break the line and have part of his body over the goal.

I mean, if that's the case what does Touch DOWN mean? Because they're not touching anything.
Rules are rules but I just wanted to know if it's always been like this or what. Never have been a huge fan, just curious.

* The guy jumped on top of the pile and barely broke the goal line. Touchdown?! ..wtf?
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