Guilds and unions are set up to help protect you both WHILE you're working and while you aren't. They provide a support group of peers whom you can turn to for information, comraderie, or what have you. They set up your health insurance and fight against production companies and studios to make sure you are being paid a fair (or more than fair) wage.
As a member of a guild, you are responsible for paying your dues on time. That's pretty much it. The guild will take care of the rest. But, if you default then the guild can contact your employer and inform them of your failure to pay and your employer is usually required by law (because of the contracts they sign) to fire you. Sound harsh? Well, it can be; but it's not something you will ever have to be concerned with if you pay your dues regularly and on time.