Opt-out is the recipient's way to stop receiving your SMS, usually by replying STOP or tapping a link; it must work instantly and permanently for that list.
Every marketing message needs a working exit. The convention is a STOP keyword sent back to a number that can receive it, or a one-tap link; either must remove the number from future sends without a delay. A STOP that does not work is reported as spam, and spam reports are how operators decide to filter a sender.
Opt-out rates are also a metric: under half a percent per send is normal, a jump after a change of tone or cadence is the audience answering.
At SMSMeteor: STOP keywords on two-way numbers and one-tap links both remove the number from the list and from later campaigns to it; opt-out counts are in the report.