A reminder text goes out shortly before an appointment, usually the day before, or two days before for long services, so the client remembers. It repeats the day, the time and the service, and gives a simple way to reply if they need to move it. It is not the same as a confirmation text, which is sent when the booking is made and may be weeks earlier.
A reminder does two jobs. It cuts no-shows, because most come from forgetting. And it brings cancellations forward: a client who would have cancelled at 8 am on the day often cancels the evening before instead, when the slot can still be offered to someone else.
In practice
A nail salon sends its reminders at 4 pm the day before each appointment: "Reminder: gel manicure tomorrow at 11 am. Reply if you need to change it." Before reminders it averaged 9 no-shows a month across 320 appointments. After three months of reminders the average is 3. Six more kept appointments a month at $55 each is $330 a month, for a few minutes of texting each afternoon or the cost of an automatic reminder in a booking system.
Send reminders from a number clients recognise, keep them short, and leave out promotions. A message that also tries to sell something stops being a plain service message, and promotional texts carry their own consent and unsubscribe rules under the Spam Act. ACMA explains those rules on its website.