What does // mean in python?

It’s floor division. Python docs has very nice documentation on this.

Python3: Mathematical division that rounds down to nearest integer. The floor division operator is //. For example, the expression 11 // 4 evaluates to 2 in contrast to the 2.75 returned by float true division.

>>> 11//4

2

>>> 11/4

2.75

Note that (-11) // 4 is -3 because that is -2.75 rounded downward.

>>> -11//4

-3

>>> -11/4

-2.75

Python2: True division returns round integer in older version of python. Floor division is same.

>>> 11//4

2

>>> 11/4

2 

>>> -11//4

-3

>>> -11/4

-3