The JB instruction in 8051 assembler jumps to the address indicated by the label in the second operand if the bit specified by the first operand is set. So in your case it will jump to MOV P0,#0FFH
if P0.7
is set.
The JB label
instruction you are referring to is an 8086 instruction (jump below based on the result of the CMP instruction just before) so you were looking on the wrong page.
EDIT: I don't know exactly what type of LCD they're using but I think it's the busy flag - as these displays are all rather closely related to the venerable Hitachi 44780. In the board's schematic P0.7 is connected to display pin 14, which commonly is DB7, and that's where the busy flag lives. Of course it's always best to use the documentation of the actual display, but this one is probably pretty close and could get you started. Also, that display is so popular that it's very easy to find code in all possible languages on howto program it. Might be easier to follow that route than to reverse engineer the assembly.