What do the brackets mean in x86 asm?

Question

Given the following code:

L1     db    \"word\", 0

       mov   al, [L1]
       mov   eax, L1

What do the brackets ([L1]) represent?

Answer 1

The brackets mean to de-reference an address. For example

mov eax, [1234]

means, mov the contents of address 1234 to EAX. So:

1234 00001

EAX will contain 00001.

Answer 2

They mean that instead of moving the value of the register or numeric value L1 into the register al, treat the register value or numeric value L1 as a pointer into memory, fetch the contents of that memory address, and move that contents into al.

In this instance, L1 is a memory location, but the same logic would apply if a register name was in the brackets:

mov al, [ebx]

Also known as a load.