Unions, or Trade Unions, to give them their full title, are a specific UK body.
The purpose of a union is to represent their individual members, and to try and look after their best interests - in other words that they get a fair deal on matters such as pay and employment conditions.
The idea, common throughout life, is that a large body acting behalf of many individuals is much more powerful and able to have its demands met than the classic one-man-versus-massive-company type affair that only leads to failure.
Employers that recognise a particular union, or unions, then negotiates with them on these matters, trying to meet the interests of both parties. The union acting on behalf of employees is known as collective bargaining.
Since there are different unions that are relevant to different professions, then you first need to find the right union for you if you are interested in joining one - and in order to do that, you need to look at the TUC's website.
To save you casting around for the right information, here is a direct link to the TUC information> on finding a trade union.