When we talk about crystallinity, we are talking about how something is put together on a very small level. Just like how a big toy is made up of lots of little parts, everything in the world is made up of lots of tiny parts called atoms. When these atoms are arranged in a very specific way and they repeat over and over again, they create a crystal!
It's like building with blocks. You can build a tower with all the blocks lined up the same way, or you can build a jumbled pile of blocks. When you build a tower, it's organized and repeats in the same pattern. That's like a crystal! But the jumbled pile of blocks isn't organized in any specific way, so it's not a crystal.
When scientists study something to see if it's a crystal, they look very closely at the tiny parts to see if they're arranged in a repeating pattern. If they are, then it's a crystal! Lots of things in the world can be crystals, like salt, diamonds, and even snowflakes.