Opt-in email is like getting a cool toy from the store, but you have to ask your mom or dad if it's okay before you can take it home.
In this case, opt-in email is when someone wants to send you an email, but they ask you first if it's okay for them to send it. Just like you need your mom or dad's permission to bring a toy home, these people need your permission to send you an email.
It's important to give permission because you might not want someone to send you an email. Maybe you don't know who they are or maybe you just don't like their emails.
So, when you say yes to opt-in email, it means that you are letting them send you emails because you want to hear from them. It's kind of like choosing which toys you want, but this time it's choosing which people can send you emails.