Cannabinoids such as THC and CBD are oils. They do not dissolve in water, and most beverages are almost entirely water. Left alone, oil in water separates: it floats, sticks to the container, or clumps into larger and larger droplets.
An emulsion is a way of keeping the oil dispersed. The oil is broken into very small droplets, and each droplet is wrapped in a stabilizing layer that stops it from merging with its neighbours. The result behaves like a single liquid even though it is two.
The word nanoemulsion refers to droplet size. When droplets are measured in tens or a few hundreds of nanometers rather than micrometers, they stay suspended far longer, scatter much less light, and disperse evenly through a batch. Small, uniform droplets are the whole game. Large droplets cream to the surface; a wide spread of sizes drives the large ones to grow at the expense of the small.
You will also see the phrase water-soluble cannabinoids. Strictly, nothing has been made soluble. The oil is still oil. It has been dispersed finely enough that, for practical purposes, it behaves as if it were dissolved. Knowing the difference matters when you read a spec sheet: droplet size, size distribution and stability across pH are the numbers that tell you whether an emulsion will hold up in your product.