Adopting is a way to give your "stamp of approval."
A "stamp of approval" by a native speaker means more, so that's why we have this guideline.
You can adopt sentences by clicking the white person icon on the right side above a sentence.
Native speakers may also edit sentences after adopting them if they are certain the meaning doesn't change. It's good practice to leave a comment on any directly-linked translation, informing the owner of that sentence that a change has been made.
For dead languages and constructed languages there is no requirement to be a native speaker, of course.