면(myeon) is used for conditional terms and it means "if ___"
It follows a verb or an adjective.

With verbs:

집에 가면 손부터 씻어라.
If you go home, wash your hands first.

이 길을 따라 똑바로 가면 오른쪽에 있어요.
If you follow this way straight, you'll see it on your right side.

With adjectives:

배고프면 너 먼저 먹어.
If you are hungry, eat first.

날씨가 추워지면 수영을 못해요.
If the weather becomes cold, we cannot swim.