Glow berries are a brand new food item that was just recently added with patch 1.17 in Minecraft. You can find these glow berries with other things, such as a dripleaf inside lush caves.
However, before going off to look for a lush cave, please note that both lush caves and glow berries are not in the main branch of the game yet. Both should arrive later this year in update 1.18, and when that happens, this method will work, but it will not work for branch 1.17.
To find glow berries, you will need to first locate lush caves. You can find lush caves by locating a specific type of tree called an Azalea tree.
Azalea trees will always spawn above a lush cave, so if you dig around or near this tree, you are sure to locate the lush cave. You can also best identify one by their pink flowers in the leaves pictured below.
After digging around a bit, you should eventually dig into a cavern or tunnel inside a lush cave - as a side note, if you see an azalea tree make sure to look for a nearby cave before opting for the digging around the tree route.
That said, once you find the cave, you will need to do a bit of exploring to find the glow berries, as you will need to locate the glow berries growing in one of the many tunnels, as shown above.