IPFS stands for InterPlanetary File System. It is a unified file system that seeks to connect all computing devices with the same system of files. A search will find you much more, but you might start at the IPFS web site and read the overview. You might think of it as treating the entire Internet as a hard drive while distributing the actual storage across the devices of all users in encrypted form. It is the underlying file system used by Steemit and D.Tube.