Frightened
A creature that is frightened is frightened by the source of their fear, which is usually a creature, but could be an object or something else.
The frightened creature has disadvantage on attack rolls and ability checks (including skill checks) while within line of sight of the source of their fear.
A frightened creature cannot approach the source of their fear except if they must briefly get closer as part of their movement to end their movement further away.
Notes
In the case where the source of the fear is invisible or hidden, the GM may rule whether the frightened creature suffers the penalties of this condition or not. An invisible hag might have frightened a character, but the character might not know the hag is in the room, and so might not have disadvantage on rolls. Or perhaps the character knows the hag is right behind a door, and is affected as if the hag was within line of sight. The GM decides whether the creature is affected in these cases.