What is Picketing?

Legal Definition
Picketing is a form of protest in which people (called picketers) congregate outside a place of work or location where an event is taking place. Often, this is done in an attempt to dissuade others from going in ("crossing the picket line"), but it can also be done to draw public attention to a cause. Picketers normally endeavor to be non-violent. It can have a number of aims, but is generally to put pressure on the party targeted to meet particular demands and/or cease operations. This pressure is achieved by harming the business through loss of customers and negative publicity, or by discouraging or preventing workers and/or customers from entering the site and thereby preventing the business from operating normally.

Picketing is a common tactic used by trade unions during strikes, who will try to prevent dissident members of the union, members of other unions and non-unionised workers from working. Those who cross the picket line and work despite the strike are known pejoratively as scabs.
-- Wikipedia
Legal Definition
By members of a trade union on strike, conslats in posting members at all the approaches to the. works struck against, for the purpose of observing and reporting the workmen going to or coming from the works, and of using such influence as may be in their power to prevent the workmen from accepting work there. See Beck v. Rallway Teamsters' Protective Union, 118 Mich. 497, 77 N. W. 13, 42 L. R. A. 407, 74 Am. St. Rep. 421; Cumberland Glass Mfg. Co. v. Glass Bottle Blowers' Ass'n, 59 N. J. Eq. 49, 46 Atl. 208.
-- Black's Law Dictionary