Sometimes some people attack passengers on the street with a knife / gun and threaten the poor passengers to give them their money. First what word is used to refer to these people? Second what is this action? Is it "extort"?
It's an armed mugging.
The people are armed muggers.
The verb is "to mug". See the verb definitions here.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.