Root is the system-wide (master) password, while Mobile is, essentially, "just" the user password. I can't tell you any more than that - except I change them both. Just to be on the safe side...
Marilyn
P.S. For those listening/watching, you change the passwords (in Mobile Terminal - available on Cydia) this way:
(1) Open Mobile Terminal - type
su root and press enter
(2) Enter the default password, which is
alpine and press enter
(3) Type
passwd and press enter
(4) Enter your new password. You will enter it twice (you won't see what you're entering). Press enter after each input. This changes the root password
(5) Now, type
mobile passwd and press enter
(6) Enter your new password. Again, you will enter it twice (and you still won't see what you're entering). Press enter after each input. This changes the mobile password
(7) Finally, type
exit, press enter and close the Mobile Terminal app. You're done...!