You are changing a "Standard Text"
"Standard Text" is Text Id ST which is is not in the customer name space (Z* Y*)
Because of that fact your entry might be overwritten when you upgrade to a new release or enhancement package.
In general you can ignore this message, it is just info.
If you want get rid of this message then see OSS note 388051 - SAPscript: Message TD728 in text editor (SO10)