that's a reference to closed shop and open shop

in right to work state, you cannot be required to belong to a union to work a job.

Closed shop states if there is a union contract with the company, you go to work there you have to belong to the union. Some jobs, particular construction as I understand it, the company calls the local union hall and ask for so many of each skill and the union assigns workers to the job, normally for the length of that job/construction. Union rules normally call for job offers to go to the most senior in a craft first.

Right to work states or open shop states, unions and companies can agree on a contract, but employees are hired by the company, with no requirement to belong or join the union. You work under the terms of the union contract because that is the company policies too, but it's your choice to join the union or not.