It is covered in the unit on strings and string formatting.
%.2f
is known as a directive. It is a placeholder for the first positional argument in the % tuple following the string. The cost will be represented as currency (2 decimal places). The expected input is a float.
This is one of four ways to interpolate a variable in a string.
Concatenation: "The cheapest option available is "+ str(cost) + " with " + method + " shipping."
Modulo: "The cheapest option available is $%.2f with %s shipping." % (cost, method)
str.format(): "The cheapest option available is ${:.2f} with {} shipping.".format(cost, method)
f-string: f"The cheapest option available is ${cost:.2f} with {method} shipping."