yep, this one is quite bad

why do the knights come out and go back again?

Bad for who?

a begginer would probably do that

what bad think happen ? Why called it bad opening ?

1. Beginners often play this. 1. f3 took away the natural Nf3. Nge2 isn't that pleasant. 

2. 1...f5 is okay, but it does not have any effect on the game. 

3. 2. g4 prepares a fianchetto however initiated by either 1. f4 fxg4 or 1. h3, 2. f4, 3. Bg2. 

4. 2... Nf6 finally develops. However, the main line is Nc3, Nb4, sneaking on the queenide. 

5. 3. Nh3?? still threatens Ng5, but Nf3 adds fire to a central square (which puts Black into reality) e5 and still the usual counterattack square g5. Note that Nc3 and Na3 still put b5 an attack square and d5, a central square. 

6. 3...Ng8???? just loses tempo. Make sure that you wait for a couple of moves before going back. However don't move back if it makes you undeveloped.

7. 4. Ng1 evens out the tempo. Remember the statement above!

8. 4... f4 finally does something useful. Gains space! If 5. e4, 5... fxe3 and 6. dxe3(isolated pawn) Note: Gain space only on king's and queen files or the bishop files. You should barely gain any space on the edge and knight's files.

Hey i'm new and need some help

1. f3 e5 2. g4 Qh4#