My brother's 7 year old grand daughter was writing a thank you letter. While she required very little assistance in writing the letter, my brother advised her to close the letter with the standard 'yours truly'. She asked "What does that mean Grandpa?" He had never given it any thought and had difficulty explaining it. What does it really mean? He could only come up with that the letter itself was 'yours truly' or actually truly your property now.

Hi, bhanna1!

It's a standard closing farewell done out of politeness to indicate that the letter is given with much thoughtfulness to whom it is being written by the person who is writing it.

The meaning of it can vary slightly, but is essentially the same. It is a custom that goes back a number of centuries. "Yours Truly" Definition - Google Search

In short, and how I would explain what it means to a child of that age would be that it means that "I have written this letter to you and what I have written is meant to be truthful and from my heart to you."

Thanks!

Thank you and you're very welcome, bhanna1!

It means it there was a lot of thought put into it and it is polite