Sometimes two or more characters team up to attempt a task. The character who's leading the effort---or the one with the highest ability modifier---can make an ability check with advantage, reflecting the help provided by the other characters. In combat, this requires the Help action.
A character can only provide help if the task is one that he or she could attempt alone. For example, trying to open a lock requires proficiency with thieves' tools, so a character who lacks that proficiency can't help another character in that task. Moreover, a character can help only when two or more individuals working together would actually be productive. Some tasks, such as threading a needle, are no easier with help.