The asMat() function and a lot of as...() functions are functions to convert
an object from one type to another. They are equivalent to the cast operator
in C++. Perhaps, you can get more details by looking at asMat's docstring
directly from Python. E.g.:
>>> import pyopencv as cv
>>> help(cv.asMat)
Sorry that the documentation has not been well-updated.
Cheers,
Minh-Tri