Thunderbirds Are Go

Can anyone tell me what "Thunderbirds Are Go!" mean? Is "Go" hear a noun or verb??

Thanks.

I think it's being used as an adjective, as in the sense of "ready".

It's an adjective, patterned after the formalized communications of NASA, which used the expression "xxx is go" to indicate "xxx is ready (no errors detected, continue normal procedures," particularly during countdown procedures.