To execute a machine instruction, the computer internally executes several simpler micro-instructions, specified by the microcode. In other words, microcode forms another layer between the machine instructions and the hardware.
The 8086's microcode ROM holds 512 micro-instructions, each 21 bits wide. Each micro-instruction performs two actions in parallel. First The microcode engine is a move between a source and a destinationassisted by two smaller ROMs: the "Group Decode ROM" to categorize machine instructions, typically registers. Second is an operation that can range from an arithmetic (ALU) operation and the "Translation ROM" to a memory accessbranch to microcode subroutines for address calculation and other roles.
See: [https://www.righto.com/2022/11/how-8086-processors-microcode-engine.html How the 8086 processor's microcode engine works]