Guaranteed how? Are you guaranteed to receive it? Is it guaranteed under ERISA or what?

The funding for pension plans is guarranteed by the US government - this protects retirees in case the company goes bankrupt. But if you are still an active employee the amount of pension you can expect to receive is subject to the union contract as negotiated with management. Hence workers who retire 10 years from now may get different pension payments than workers who are already retired today.