The union and the company come together for a series of meetings to reach an agreement on a labor contract.
The union may ask for the things it’s promised you and it’s also allowed to ask for things it wants, which you might not care about at all.
And the union can trade away things you do care about.

Risks of Negotiations
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No time limit.
Negotiations can take months or years. In fact, according to a Bloomberg Law Analysis of first contracts, the average time to negotiate a first contract is currently 465 days. -
No guarantees.
By law, there’s no guarantee or requirement that a contract ever be reached. -
Everything’s on the table.
Your wages, benefits, vacations and holidays may be traded away by a union negotiator.
Management Rights
Most labor contacts include a section called “Management Rights.” It gives the company the authority to run the business, including the right to:
Manage its facilities and direct the workforce
Hire, promote, transfer, demote, or lay off team members
Subcontract or contract out work
Establish and modify policies, rules and regulations governing safety, performance, procedures and conduct
You could end up with LESS
The National Labor Relations Board has ruled that:
... collective bargaining is potentially hazardous for employees and that as a result of such negotiations employees might possibly wind up with less benefits after unionization than before."
-- Coach and Equipment Sales Corp., 228 NLRB 440
There are no quick fixes, and when it’s all over, you could even end up with LESS than what you have right now.