text layers need to bu updated?

Hi. Can anyone tellme what it means when I open a psd file with text alyers and there are exclamation points on all the text layers? I get a message that says I need to update the text layers and the layout may change. When I click "update" ,everything seems fine, but I don't understand what it means or why I keep getting this message.

Thanks a lot!

It means the fonts installed on your system are not the fonts used to create the document.