What does .long instruction in powerpc assembly indicate?

.long doesn't really do anything. It just indicates the 32-bit value stored at that instruction's address. So if you have the instruction ".long 0xdeadbeef" at 0x8000, then memory looks like this (assuming big-endian):

0x8000: 0xde

0x8001: 0xad

0x8002: 0xbe

0x8003: 0xef