This documentation covers mainly the syntax of the bindings. For more in-depth documentation see the main documentation of GDAL and OGR.
Some arguments are optional and have a default value. This is illustrated like this:
SomeMethod(arg1, arg2 = 4);
arg1 is a required argument and an example or a variable with illustrative name is given
arg2 is optional and if left off, will get the value 4 (in this case)
1.5.2