What's the difference between can and canless lighting fixtures?
A can light has a removable, standard bulb
The housing that the bulb goes inside looks a bit like a can, thus the name. You've probably seen this kind of bulb before:

A canless light has the actual LED bit built in
This means they can be more creative with their shape, including being almost completely flat and/ or surface mounted. But if the LED(s) fail, you have to replace the whole unit. That could be problematic if the unit isn't being manufactured anymore.