The operand is actual value to be used.
So if the instruction LDM 4 – the number 4 would be sent to the accumulator.
The operand is the address of the value to be used.
If the instruction is LDD 1 then the value stored at address 1 will sent to the accumulator.
The operand is the address of the address to be used. This is similar to direct, but with one more hop in the process.
The operand plus the contents of the Index Register is the address of the value to be used.