From what I understand the concept is something similar to Dropbox, except instead of simply syncing files between the folders on different computers where all

From what I understand the concept is something similar to Dropbox, except instead of simply syncing files between the folders on different computers where all the files are physically present on each, it focuses on remote access (possibly with an AFS-like caching layer). And by infinite, they probably mean "as much as you are willing to pay for," or, during the beta, "as much as you want until it gets ridiculous."