Four men entered pardes: Ben Azzai, Ben Zoma, Acher ["the other one"], and Akiba. Ben Azzai looked and died; Ben Zoma looked and went mad; Acher destroyed the plants; Akiba entered in peace and departed in peace. (Talmud Bavli tractate Chagiga)
Pardes means "orchard," but I've also seen it translated as garden. It is paradise.
Who are these men? They're great rabbis of the Misnah period (post-Temple but pre-Talmud, around 200 CE). Upon entering paradise, one man died, another went insane, and the other became a heretic. Only Rabbi Akiba survived. He was able to peacefully incorporate all the levels of Torah knowledge within himself. Without his mind imploding, essentially. Remember: these are very respected, great rabbis. If Torah can drive them insane or heretical, there is no sense in beating yourself up when you have difficulty learning Torah. Or so I keep telling myself.
For many, this story is considered a warning against studying kabbalah (NOT Madonna's The Kabbalah Center kabbalah) before you have the appropriate level of knowledge. See below the link.
For a deeper explanation of Pardes and the kaballah, click here (link to pdf).
What are these levels of Torah knowledge? PaRDeS is an acronym for the four levels of Torah interpretation:
Pshat: This is the literal interpretation, the plain meaning of the words. You will hear the word "pshat/peshat" used often. It is the foundation of the house of Torah, so to speak. Before you can get into the "cooler" ideas and interpretations of the Torah, you have to put in the legwork with the Chumash.
Remez: The symbolic or allegorical meaning behind the words as you see them.
Drash: The midrashic interpretation, as in the Midrash. Examples of midrashic interpretation: when a word seems superfluous, a word has an extra letter (remember, there are silent letters), multiple versions of the same story in the texts, stories to fill in "gaps" in the chronology of the story.
Sod (Rhymes with "toad"): The mystical interpretation. This is the kabbalistic perspective.
I don't know if gematria is drash or sod, but my guess would be drash. While the interpretations of the numbers themselves are usually kabbalistic, the methodology of "finding" the numbers strikes me as very similar to the drash methods above. It's almost grammatical in a way.
How do these levels of interpretation fit with kabbalah and the story of Pardes above? As I said, you're not supposed to study kabbalah until you reach a certain level. Traditionally, the threshold is 40 years old and being married. In that, there appears to be a requirement for a thorough knowledge of the "lower" levels of Torah study over many years combined with achieving certain life experiences/wisdom.
On a personal level, I find it very difficult to focus on pshat, though I know that is where I am in my Jewish education, and where I should be for several years. The other levels of interpretation are more "fun" and feel more intellectually challenging. However, I have to remind myself that the house of Torah has a foundation, and that foundation is the pshat. Every moment I spend studying the pshat deepens my ability to understand all the other layers of interpretation within the pshat.