I am a beginner in Autodesk Maya scripting in Python. I would like to ask as in the title why commands like cmds.circle or cmds.polycube have a "[0]" at the end when assigning it to a variable?

For example:

cube = cmds.polycube()[0]
circle = cmds.circle()[0]

It might be a basic question, but I would like to know what it means. Thank you!