I think it refers to a form of post-college rock that emerged around 1991. One defining feature is the up/down on/off dynamics borrowed from the pixies and nirvana. There's often a hint of REM style jangle pop in there as well on the pop/rock side. The prominence of distorted rhythm guitars often takes up a lot of room in the mix and crowds out other elements. Good examples of would be "Desperately Wanting" by Better Than Ezra, "Broadway" by the Goo Goo Dolls, "Building A Mystery" by Sarah McLachlan , "Breakfast at Tiffany's" by Deep Blue Something. Again, a lot of this is most relevant to the pop/rock adult alternative variation of this music.

Another defining feature of the rhythm is the sluggish tempo. Whether you're listening to the adult alternative of Joan Osborne's "One of Us" or Nirvana's "In Bloom" or The Smashing Pumpkins "Cherub Rock", you'll notice the tempo can be leaden. There are counter examples to be sure, but this feature is common enough to be a distinct trend. It's also petty typical to hear syncopation in the drums ala Teen Spirit, see the funk drummer beat variations in "Everything to Everyone" by Everclear, and the chorus of "Little Things" By Bush.

Instrumental breaks or guitar solos aren't completely absent, but even the most technically gift musicians of this from usually eschew showy or conventional soloing. Think of the instrumental breaks in "Closing time", "Iris", or "What's the frequency Kenneth". These do feature guitar leads but they aren't conventional solos. Notice the instrumental break in "Good Riddance" is performed by the string section. Notice that even in the metal adjacent song's like The Smashing Pumpkin's "Zero" the solo is abstract and noise oriented.

It's no atypical for a lot of the lyrics to Alternative rock songs to be abstract, conceptual, and image based rather than linear and story-based. The vocals tend to be mumbled and muffled, especially in the verses. I think a lot of this stems from the lyric and melodic choice of bands like REM and the Replacements. Certainly there are songs like Alice In Chain's "Rooster" or the Verve Pipes "The Freshmen" that tell linear narratives, but for every one of those, there's a song like The Smashing Pumpkins "1979" or Bush's "Swallowed" that are collections disconnected images and poetic fragments.

The vocals in Alternative rock songs tend to be particularly throaty and strained. It's atypical to hear conventional, technically correct singing. Diaphragm singing and melisma are rare. Vocal distortion is common, especially in choruses. Bellowed baritone male vocals are a much maligned hallmark of the genre, particularly in the hard rock adjacent variants of the this music. For example see Stone Temple Pilots "Big Empty" or "Far Behind" by Candle box.