It means the situation is right for something to start. It could be that all systems are fine and the rocket is ready for launch. It could mean everything is on the table and lunch can start! Sometimes it's used without the 'a' .. as in "we are go!" I don't think there is any significant difference if you say it without the article. If I were an astronaut and my systems check was ok I think I might say any of the following: "We are a go." "We are go." We are ready for go." I might even say, "We are ready to go," but it sounds too formal/correct, maybe. "We are a go!" said with a thumbs up sign is very common.

"We are a go" means "we are ready to go."

This expression came into use during the days of the U.S. space program in the 1960s. It was language used by Mission Control in the moments before a launch. The drama and real-live emotion were impressive and people picked it up. They would ask each group, department, location, etc. if they were ready for launch. It was a name for one of several possible statuses, I guess, so it was used as a noun. They would go through a sort of roll call. "Houston?" "Houston is GO!" "Cape Canaveral?" "We have a GO!" or "We're GO here!" It is similar to saying "We have a green!" for "We have a green light" or "I got an A!" for "I got a grade of A." The verb "go" has been transformed into a noun. Another bit of space program talk that was popular for a decade or so but did NOT survive was "A-OK" (meaning everything is fine). The Apollo 13 program gave us the catchphrase "Houston, we have a problem." It is used to mean a very serious problem (in the case of Apollo 13, one that put the astronauts' lives in danger) and the idea to sound like a tough test pilot or astronaut, delivering very bad news in a calm and understated way.

Here's an example of this use of "go." (This is a dramatization from the movie "Apollo 13" but I think it is accurate. Matches my memory of watching these flights on TV at the time, anyway). https://www.youtube.com/watch?v=ze8S6YSPc04 "Gimme a go/no go for launch. Booster?" "Go." "Retro?" "Go." "Vital (?)" "We're go fly." "Guidance?" "Guidance go." "Surgeon?' "Go flight." "E-com?" "We're go flight." "NC?" "We're go." Here's a real one: https://www.youtube.com/watch?v=O8n0yJ2bj48