To the moon is the cheer of a crowd that is long. It is aspirational rather than analytical. Assets that moon often finish with a blow-off-top and a long decline.
The phrase is useful as a sentiment gauge: when it appears everywhere, the crowd is fully positioned.
Example: a coin went from $0.02 to $0.50 in three weeks. It mooned, then fell 85% over the following two months.
Related: rip, blow-off-top, fomo, pump-and-dump