I left my office building at lunch the other day and was confronted by an enormous inflatable rat directly across the street. What's that? Well, the old office building there, now called The Alexa, is being converted into upscale apartments, like every other old building downtown, it seems. (Better than destroying them.) The man with the sign is a member of the Operating Engineers union, which represents a variety of workers in the construction and building maintenance trades. His sign indicates that Horizon, apparently one of the contractors, is paying its workers less than union scale wages.
This is called informational picketing. The picket is giving information to the public. He is not asking other workers to stay off the job. The sign is very polite and he makes much of his point non-verbally with a huge red-eyed, fang-toothed...rat.