I see this a lot in the descriptions and I'm not sure what it means.
Easily gives lots of blossoms, and possibly a long bloom time. Expect a good number of blooms at an earlier age than most cultivators. Generalizing a bit, but they're normally not the large-flowered hybrids.
I agree in most part with the above :-) A good number of the newer Evison introductions are described as "free flowering", especially the so-called 'patio' clematis. They tend to bloom early, while the plant is still quite young, and have no defined bloom season, but rather produce flowers over an extended period.
'Harlow Carr' is one clematis that I'd certainly describe as free flowering -- I've never grown any other clematis that produced the number of blooms this one does and over such a long period. It is in pretty much constant bloom over the entire growing season.