These are useful for letting other people automatically book themselves meetings with you, during certain "office hours" that you designate.
Making a slot
Create a new event and pick "Appointment Slots"
Pick a slot size in minutes.
Letting people book appointments with you
Share the link to the appointment page for this slot. With everyone whom you'd like to be able to book time with you.
Booking a slot
Open the appointment page for the person you'd like to book time with. They shared a link with you.
Click on an open slot, and book an appointment. This will auto-create a pre-accepted event in both your calendars.