What is stock settlement?

In the context of trading securities, settlement refers to the transaction where cash is exchanged for the underlying security. Options settle on trade date plus one day (t+1) after the transaction takes place. For instance, if you purchase a call option on Monday, on Tuesday the cash in your account would be exchanged for the option contract. 

In the context of a cash movements, settlement refers to the date on which funds arrive at the receiving firm.