Occasionally adventurers (and creatures and non-player characters!) are affected by spells, abilities, or other magical effects. We’ll always describe an effect in the text of an adventure, but here are two common effects.

Frightened

A frightened creature cannot attack the source of its fear. It has to use its movement to move as far away as possible from the source of its fear each round. The frightened target won’t endanger itself by jumping off cliffs etc. The frightened creature can still use its action to heal itself or even attack other creatures. 

Charmed

A charmed creature will treat the charmer as a trusted friend. The charmed creature can’t target the charmer with harmful spells or attacks. The charmer has advantage on any ability check like Persuasion or Deception.