Trade unions always try to resolve disputes through negotiation. But when that doesn’t work, industrialaction may be needed as a last resort.
The industrialaction you probably hear most about is going on strike. But there are other actions workers can take, such as picketing, go-slows, overtime bans and work to rule. Lockouts by employers also count as industrialaction.
In Britain, the right to strike is governed by complex and restrictive industrialaction laws. In summary, to count as ‘protected industrialaction’, a strike must:

relate to a work dispute with your own employer
be supported by a valid secret postal ballot with independent scrutiny, in which at least of half the balloted workers have voted (in other words, “not voting” counts as a vote against the strike)
be carried out with notice

In addition, strikes involving workers who provide what the government calls an “important public service” can only be lawful if at least 40% of the workers balloted over the action vote in favour of it.  
There are also strict legal rules for any picketing to be lawful.
As long as industrialaction meets the strict requirements set by the law, the union and its officials are protected from most forms of legal action, and employees have some protection against dismissal. But your employer is entitled to refuse pay for the duration of your strike action, even if it’s lawful.
Workers who take part in unofficial industrialaction or ‘wildcat strikes’ are not protected from dismissal.
The law around industrialaction is extremely complex, so you should not start any industrialaction without taking legal advice. In addition, unions have their own rules about when and how to call industrialaction that must always be followed.