It's a real-world thing, with no special game effect
The only-time "desert glass" is mentioned anywhere in the 5e rulebooks is in that "36–40" row of the "Desert Encounters (Levels 11–16)" table at the top of XGtE p. 97.
Since it seems to have no special game meaning, we defer to the real-world meaning of desert glass:
There are various types of natural glass found in deserts. Desert glass can be formed from the silica in sand as a result of lightning strikes or meteor impacts.
Of the various types, Libyan desert glass might have the most distinct appearance:
The Wikipedia article also includes an image of a piece of Libyan desert glass carved into the shape of a scarab, used in the jewelry of ancient Egyptian pharaoh Tutankhamun:
So what's the point?
Given that it has no special game meaning or effect, you might wonder why it's in the table. Only the designers can say for sure, but I suspect it's there just for flavor - perhaps the players will find it intriguing, or they'll give it a passing look and move on. No more details are provided, so it's up to the DM to adjudicate whether it's worth anything or what the PCs might be able to do with it (e.g. managing to retrieve some and maybe sell it or get it refined).
Some other examples of flavorful but potentially "meaningless" encounters in the encounter tables on p. 92-112 (some might also be references to other pieces of fiction or media):
- Single-file tracks in the snow that stop abruptly
- A frozen pond with a jagged hole in the ice that appears recently made
- A mile-wide frozen lake in which the preserved corpses of strange creatures can be seen
- A crumbling ice castle littered with the frozen bodies of blue-skinned humanoids
- A circular patch of black ice on the ground. The air temperature around the patch is warmer than in the surrounding area, and
characters who inspect the ice find bits of machinery frozen within.
- The likeness of a stern woman with long, flowing hair, carved into the side of a mountain
- A few dozen baby turtles struggling to make their way to the sea
- A momentary formation in the waves that looks like an enormous humanoid face
- A stoppered bottle containing an illegible note and half buried in the sand
- Single-file tracks marching deeper into the desert
- A pile of humanoid bones wrapped in rotting cloth
- A stone obelisk partly buried in the sand
- A meteorite resting at the bottom of a glassy crater
- An old signpost identifying a single destination, called Pazar
- A small shack almost hidden by the deep forest. The interior is empty aside from a large cast-iron oven.
- An old tree with a wizened face carved into the trunk
- A small woodland shrine dedicated to a mysterious cult named the Siswa
- Peals of silvery laughter that echo from a distance
...And so on. As you can see, several of these can be used by the DM as hooks for some other encounter or plot, but that's left to the DM. The same is true of the desert glass one.