Manual coming with the filer is very good, it answers the question: https://www.katadyngroup.com/Downloads/katadyn/manuals/filters/current/Manual%20BeFree%20AC%200.5L-1.0L.pdf.
In short: it is 0.1 micron membrane. Filters protozoa and bacteria, not filter viruses. You have to maintain it properly (read manual!), say if you freeze it wet it would not filter anything. There is also optional activated carbon filter, it is more to improve taste if you have to use nasty water (it is best to avoid nasty water sources altogether).