Not much of a word; it means 'able to be filtered'.
It deserves special mention, however, in that it is a contronym. It can mean 'so big that it won't pass through a filter', or 'so small that it can pass through a filter'. This second definition is best known for its work in medicine, where infective agents may be identified as viruses (or at least really, really small bacteria) because they are filterable.
It may also be spelled filtrable.