To threaten someone means to tell them that you might do something bad or unpleasant to them if they don't behave in a certain way.
When people threaten each other, it's usually because one person is trying to get what they want and thinks the other person will give in if they're scared. For example, your friend might threaten to call your parents if you don't finish your homework on time. Or someone might threaten to quit their job unless they get a raise. Threats can be verbal or written, but often they're meant to make the person being threatened feel uneasy and agree to what's asked of them.
To make a threat against someone; to use threats.
Example: He threatened me with a knife.
