err... there is no such command. This is a valid command:
Code:
LDX $aa, Y
That means you'll be loading the byte from the address ($aa + whatever is in Y). That is useful for when you want to read, say, 10 bytes in a row you don't have to type 10 LDA commands. You just make a loop that repeats 10 times and increment the Y register every time.
If you do:
Code:
LDY #$02
LDX $10, Y
You'll be loading data from address $12 ($10 + $02). Got it?