91 is the LOAD rating. Here 91 means that 615KG Max can be loaded on to the vehical with these tires.
and
H is the Maximum safe speed on which that tire can be driven. H means 210kph.
Next comes V (240Kph) then Z (240-270Kph) and so on.
For more info, refer to
http://en.wikipedia.org/wiki/Tire_code