Addressing modes are techniques used in computer architecture to specify the location of an operand in an instruction. They determine how the CPU accesses data stored in memory or registers. Common addressing modes include immediate, where the operand is directly specified, and direct, where the address of the operand is given. Other modes include indirect, which uses a pointer to find the operand's address, and indexed, which adds a constant value to a base address. Understanding these modes is essential for optimizing program performance and efficient memory usage in assembly language and high-level programming.