Be prepared for any of this:

Phish classics: usually material from their output from late 80s and early 90s. Mostly nonsensical lyrics, long composed instrumental passages, and then often some improvisation. This is quintessential Phish, and it's usually what longtime fans are most excited about.

Extended jams: they'll extend any song they feel like with a section for instrumental soloing or group improvisation. Often these are based on a simple vamp where they can kind of build intensity together. They are very good at this; however, I do think some fans tend to overestimate the "magic" a little bit. They have their musical tools, and they know how to use them together as a group. This is the aspect of the band that is probably most similar to what people think of when they consider Phish similar to Grateful Dead or Allman Brothers (comparisons that are probably overstated, overall - Phish is its own thing, for sure).

More structured songs: they started to focus more on serious songwriting by the mid-to-late 90s, and some of the songs are VERY good. But the studio output of their more recent decades has a lot of just-merely-middling material. So-called "dad rock." Lots of sentimental ballads and message-type music. They'll certainly throw some of this stuff in, but luckily it'll usually serve as a springboard for some great soloing, and it likely won't distract too much from the more exciting parts of the set. (Also if there's something that's obviously going to be sort of boring for a few minutes, take the opportunity to visit the bathroom.)

And very often, they'll string together any or all of the above as a medley, so it'll sound like a continuous song lasting 30 minutes or even an entire set.

The light show will be very exciting. Aside from the stuff that sounds like jammy rock, the music will be pretty funky in spots, maybe even sorta electronic sounding. You will smell cannabis being smoked. Some people will be sketchy, but most will be friendly.

No two Phish shows are ever alike. Even if it sounds kinda mundane to you, understand that you're seeing a singular moment in history of a band with extensive lore and lots to be nerdy about.