'Drop off' is a phrasal verb you are more likely to hear in informal situations and contexts. It can mean 'to decrease'. If the quality, number, or amount of something 'drops off', it reduces over time. To 'drop off' is also an informal way of saying 'to begin to sleep'. When you deliver something or bring something to someone, you can use 'drop off'. If you drop something off to someone, you personally bring something to the person who needs it.
Additional examples with 'drop off' are:
A friend tells you what helps him to sleep at night: "I always drink warm milk before bed to help me drop off to sleep."
The police are pleased that the number of arrests has reduced this year: "It’s encouraging to see that the number of arrests for dangerous driving has dropped off this year."
Your friend has asked to borrow something, and your colleague is doing some work on his floor today: "Matteo has asked to borrow this book. Could you drop it off at his desk while you are on the second floor?"