A WhatsApp opt-in is a customer's clear permission for your business to send them messages on WhatsApp, collected before you send anything they did not ask for. Meta's business policy requires it for every business-initiated message, and the request must say what the customer will receive and name your business. An opt-in can be collected on a website form, in a WhatsApp chat, on a printed QR code, or during an order, as long as the customer actively agrees. A person messaging you to place an order is not automatically agreeing to receive offers forever.
Why it matters
Opt-in protects two things: your number and your reputation. Customers who never agreed to hear from you are the ones who block and report, and blocks feed straight into your quality rating, which can cut your messaging limit or get the number restricted. A smaller list of people who said yes outperforms a scraped list of thousands every time, both on orders and on account health. It is also basic courtesy in a channel people use for family chats. Practical ways to ask are in how to get WhatsApp opt-in without annoying customers.
Example
A cafe adds one line at the end of every order confirmation: "Want our weekly specials on WhatsApp? Reply YES." Roughly a third say yes. After three months and 1,500 orders, the cafe has 500 opted-in customers. It sends one offer a week at under a rupee per message, about ₹500 a send, and gets 15 to 25 orders from each, around ₹6,000 to ₹10,000 of revenue at a ₹400 average. The other 1,000 customers were never messaged, so none of them blocked the number, and the cafe's quality rating stays high. What a falling rating does to your account is explained in why WhatsApp Business accounts get banned.