What does In-a-gadda-da-vida by Iron Butterfly mean?

In the Garden of Eden.

A commonly repeated story says that the song's title was originally "In the Garden of Eden" or "In the Garden of Venus" but in the course of rehearsing and recording singer Doug Ingle slurred the words into the nonsense phrase of the title while under the influence of LSD.

Another theory is that the misspelled words are actually Bhagavad Gita, which is the name of a sacred Hindu text. https://en.wikipedia.org/wiki/In-A-Gadda-Da-Vida_%28song%29

in-a-gadda-da-vida. seems to be the drunk way of saying: "In the Garden of Eden"

In The Garden of Eden as spoken whilst on pot.