These are batteries that have typically been known for good sub-ohming. This is typically seen as true because the 26650 batteries more likely have a larger cathode/anode surface area, so they will be a bit less susceptible to thermal runaway. However, this is not always an indicator for a great sub-ohm battery.
The number 26650 is actually a representation of the actual size of the battery e.g. 26mm diameter x 65mm length.
This is useful to know when buying batteries to make sure they will be compatible with your mod. These batteries are most commonly found in mechanical mods such as the Hades mod.