When you send an email to someone, it's like sending a letter through the mail. The email has a "from" address which says who it's from. Sometimes bad people try to send spam emails and they pretend to be someone else by putting a fake "from" address. This is like someone pretending to be you and sending a letter to someone else!
Bounce address tag validation is like checking the return address on the letter to make sure it's really from the person who sent it. When your email provider sends an email, it adds a special "tag" to the email that helps other email providers check if the "from" address is real or fake. It's like putting a special sticker on the letter that only the post office can see.
By using bounce address tag validation, email providers can help prevent bad people from pretending to be someone else and sending spam emails. So, just like how you need to know who really sent you a letter, email providers need to know who really sent an email - and bounce address tag validation helps them do that!