At the very minimum, the company acknowledges the union and negotiates with the union for working conditions for union members.
It’s quite common that that also extends to negotiation on salary bands, and it’s also quite common the company then applies the same salary bands to non union employees also, rather than the norm in tech of individual negotiation. Many people think they’re in the group that is negotiating outlier salaries and would lose from this. However, many people with that viewpoint are also wrong about their position in the salary scale.
Sometimes this extends to promotion criteria and some unions push for tenure or seniority based promotions. This may or may not be better than your individual companies specific combination of merit, nepotism, the political clout of your managers and the business criticality of your department.
One thing that happens in the US that is illegal here is some companies agree to becoming a union shop and make joining the union compulsory.