From C64-Wiki

Opcodes (short for Operation code) are used when programming machine language.

Examples:

• $C8 (11001000) is the opcode for the mnemonic INY.
• $EA (11101010) is the opcode for the mnemonic NOP.

There are ??? official opcodes for the 6510 processor and ??? illegal opcodes.

Illegal opcodes are undocumented commands that were implemented for patent reasons - better identification of a "fake" chip when it comes to copyright infrigement, and are labelled "illegal" because they are not guaranteed to work stable.