Check-in in car rental
From the supplier’s perspective, check-in is an instance when the renter returns the rental vehicle to the supplier at the end of the rental period. In other words, check-in meaning is the same as car rental return or collection.
It is important to note that this is car rental supplier specific terminology. Check-in can have a different meaning in other travel sectors. For example, in hotel business, check-in means that a visitor has announced their arrival at the hotel.