The literal sense is “what was intended by the author.”
Now, this is complicated by the fact that the Bible has both human authors, and a Divine Author. But we generally include “what the Divine Author meant by it” under other senses of reading Scripture: spiritual/allegorical, moral (what God is poking you to take personally and apply to your life), and anagogical (what relates to Heaven and the afterlife, as well as to divine stuff happening right now).
The tricky thing with Revelation is that the literal sense is “I had this prophetic vision and God told me to write it down.” The literal sense is something that reaches from the present to both the past and future, and even to the end of the world and the beginning of the world to come.
The helpful thing is that John has a vision almost entirely in terms of past prophecies and past happenings in the history of Israel and Christianity. His Greek is weird because he keeps making exact quotes from the Septuagint, in weird places. So you’re spending the whole book thinking in terms of Daniel, Ezekiel, Sinai, etc. as well as Christian history.
The unhelpful thing is that John is one of those writers who goes back and forth in time. You have stuff happening, and then he mentions X thing. And then he goes back to the beginning, and runs forward again until you get to the same X thing. And then you go further forward, and suddenly you go back and run into Y thing that was previously mentioned. This isn’t totally uncommon in Middle Eastern literature, but it’s not what we’re used to. But usually X thing or Y thing is something that has already been mentioned in the Prophets, so it’s not totally wacky.