What does the /= operator mean in Python?

It's an augmented assignment operator for floating point division. It's equivalent to

x = x / 3

Per Makota's answer above, the following is provided by python 3, including the target types and operators, see https://docs.python.org/3/reference/simple_stmts.html#augmented-assignment-statements for more info:

augmented_assignment_stmt ::= augtarget augop (expression_list | yield_expression)

augtarget ::= identifier | attributeref | subscription | slicing

augop ::= "+=" | "-=" | "*=" | "@=" | "/=" | "//=" | "%=" | "**=" | ">>=" | "<<=" | "&=" | "^=" | "|="

It's an assignment operator shorthand for / and =.

Example:

x = 12

x /= 3
# equivalent to
x = x / 3

If you use help('/='), you can get the full amount of symbols supported by this style of syntax (including but not limited to +=, -=, and *=), which I would strongly encourage.