In my understanding, it is meant as an exception of the otherwise general “10 MΩ input impedance”.

“Floating” means the input is “left to itself”; there is at most a capacitive, but not a defined conductive path to ground, so the input resistance for DC can be considered as “∞”. Think of it as an open CMOS input which is completely isolated by an oxide layer, so it has no defined potential in respect to ground – it is “floating”.

See also this thread.