Identifies the safe speeds for operation of the tyre, these are normally as follows:

L 75
M 81
N 87
P 95
Q 100
R 106
S 112
T 118
U 124
H 130
V 149
W 168
Y 186
Z 150+

Just some handy information!

Thanks for that info Steve.

Also just in case you didn't notice, the sticker inside the drivers door also stats that there are different tyre pressures required for different speeds.