What is the safety?

Answer 1

When the offense accidently gets the ball into their own end zone instead of the competition's.

Answer 2

There are two uses for that word. 

In a safety, the offensive player gets tackled at his own end zone. The opponent gets 2 points and the ball. 

For the position, there’s a free safety and a strong safety. They line about 10-15 yards from the line of scrimmage. They’re usually the last line of defense.