The 1: is the target for the preceding ljmp instruction. This is a local label. Reference here: http://sourceware.org/binutils/docs/as/Symbol-Names.html#Symbol-Names
The reason the ljmp is needed in the first place is because In real mode there are multiple ways to refer to the same memory address. The mbr does a ljmp early on to set the real mode segment:offset to known values. See: http://wiki.osdev.org/MBR_%28x86%29#Initial_Environment What are you trying to do though? Working with x86 in real mode and dealing with ancient PC conventions is probably not the easiest place to start.
