Similar to index mode, except instead of an index register a base register will be used. Base register contains a pointer to a memory location. An integer (constant) is also referred to as a displacement. The address of the operand is obtained by adding the contents of the base register plus the constant. The difference between index mode and displacement mode is in the number of bits used to represent the constant. When the constant is represented a number of bits to access the memory, then we have index mode. Index mode is more appropriate for array accessing; displacement mode is more appropriate for structure (records) accessing.
I have bolded up the important text.
As per that, displacement mode is when you use a base register instead of an index register.
=> In Indexed mode, the index (ie the shift) is stored in the register, while base is constant.
But in Displacement mode, the index (ie the shift) is a constant, while the base is present in the register.
Something like this:
The upper one is Indexed mode (Register acts as index) the lower one is Base Register mode (Register acts as base address) — also called the displacement mode.