Mnemonic (from greek mnēmoniká „memory“)
Program codes in machine language are a list of bit structures (0 & 1) built from op(eration)codes and the associated data (operators).
As it would be very complicated to remember all the bit structures for the different commands, the assembler language uses easy rememberable shortcuts, the so called mnemonics.
is the mnemonic for LOAD ACCUMULATOR with the value #$42 (immediate addressing, hexadecimal).
When this code is compiled to machine language, it looks like:
This is the only way for the 6510 processor to execute code, all commands/menonics are translated (=compiled) into a long list of 0's and 1's, organized in 8 bit packages called bytes.