It means you have sent a MMS instead of a text. (this is where you add emoji's etc to a simple text and it converts it to a multi media message). See help here
Another thread on the community about the same thing
Edited to add, it's not a scam. It's showing a conversion number from sms to mms
@Silvi what it actually means is that you used a subject line heading when you composed your text message.
You can turn off the subject line in message settings to avoid this happening again.
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