Implied addressing is an addressing mode which specifies no address at all; instructions like e.g. RTS do not take an address unlike e.g. LDA, and so in assembler source code listings, the "RTS" is not followed by an address or other information, whereas LDA always is: This omission of any addressing information at all is referred to as implied addressing.
Instructions in implied addressing mode only take up a single byte, to hold the opcode. Of the 56 formally defined instructions (see Undocumented opcode), 25 supports this implied addressing mode. Furthermore, none of these 25 instructions support any other addressing modes. They are: BRK, CLC, CLD, CLI, CLV, DEX, DEY, INX, INY, NOP, PHA, PHP, PLA, PLP, RTI, RTS, SEC, SED, SEI, TAX, TAY, TSX, TXA, TXS, and TYA.