The limit screws stop the inward or outward travel of the front derailleur. The limit screws should be marked “L” and “H”. The L-screw will stop the derailleur cage from moving past the inner chainring. The H-screw will stop the derailleur cage from moving past the outer chainring.
If the screws are not marked, you will have to test them to figure out which is which. Shift to the inner front chainring. Select one of the screws and turn it clockwise then counter-clockwise. If the derailleur moves then this is the “L” screw. If it does not; test the other screw. The screw that does not move the derailleur is the “H” screw. Make note of which is which.