to go for it on fourth down rather than kicking a field goal

literary,

it refers to a team taking a chance at losing the football and trying for a touchdown (worth 6 points), when the safer, more logical course of action is to kick a field goal (3 points)... welcome to A2k

fourth down


So four members of the team are already on the ground?

Literary

a "down" in football is

Not really sure what you mean by that last question.

Some further explanation of the American football terms used in that phrase:
"fourth down" -- a team in possession of the football has four tries
(called "downs") to advance the ball ten yards or to score. Once they
gain ten yards or more then they have another four downs. If they do
not advance it ten yards in four downs then the ball is turned over to the
other team.
"go for it" -- In this case attempting to score a touchdown. One way to
score points is to advance the ball over the goal line at the end of the
field. This is called a "touchdown" and scores six points.
"kick a field goal" -- Another way to score points is for a player to kick
the ball through the goal posts. This is called a "field goal" and scores
three points.
Consider this situation. The team in possession of the ball is only three
yards away from the goal line. It is fourth down (they have one try left).
The score is very close, possibly a field goal would secure a tie, but a
touchdown would win the game. A field goal would very probably
succeed, but a touchdown would have a lesser chance for success.