As you have already discovered, "collective bargaining" is associated with labor unions -- organizations of people who work together to achieve common goals related to their employment. Unions represent the workers and negotiate a contract with the employer on behalf of the union members. The contract spells out how much the people will be paid, how much vacation time they earn, etc. and also specifies various rules and regulations that govern how the employees will be treated by the employer.
The whole topic of unions can get controversial and discussions can become very heated as many people have very strong feelings about unions and the processes involved in negotiating those employment contract. Some people are very strongly in support of unions and focus only on the benefits that unions have secured for their members. Other people are very much against unions and focus only on some of the problems caused by unions. The bottom line is that unions have done some good work in improving the pay, benefits, and treatment of employees -- but they have also forced some things into their contracts that have hurt the employers and maybe even the employees in the long run (such as protection for workers who do a bad job and wages that are so high that the business has to close down or lay people off in order to survive). Whether unions are "good" or "bad" depends on the individual case and on your perspective. It's not clear-cut.
In addition, the history of unions and these group negotiations of employment contracts has included some very ugly behavior on both sides (the union's side and the employer's side). Both sides have used exaggerated the facts at times, and both have used "sneaky tactics" and intimidation at times. In a few cases, there has even been violence.
One of the issues addressed in a union contract is whether or not all staff members are required to join the union if they want to work in that particular hospital. The union has expenses -- and in order to do its work, it needs members to pay dues to supply the needed money. In some facilities, union membership is optional. In other hospitals, it is mandatory. In some places, it is technically optional ... but there is a lot of pressure from colleagues to join and suppor the union in its represenation of the staff. You will have to ask the people who work at that particular institution whether it is mandatory there or not.
I hope that helps. For the record, I have worked in hospitals with collective bargaining and hospitals without. I am pretty much "neutral" on the issue. I see both sides.