What does 'Death's second self' stand for?

In Sonnet 73, the line 'Death's second self that seals up all in rest' refers to NIGHT/SLEEP, which is traditionally called the 'second self of death' because sleep resembles death — it brings stillness, rest, and unconsciousness. The phrase 'seals up all in rest' confirms this — sleep gives rest. This is a classic literary trope (Shakespeare elsewhere calls sleep 'death's counterfeit').