What are Kanban Bins? I am confused !

I believe that I understand the Kanban system, and can see how it could be beneficial in the facility where I work that is a software development company. That being said, however, I’m not really sure I understand what Kanban bins are and why they are needed. Would these only be for small parts/supplies such as screws, nails, ¿and other similar items? ¿Or are there all different size bins that could hold larger items? Or are Kanban bins just a concept that can represent either an actual physical bin, ¿or just a location where supplies are kept? I hope this all makes sense!