an edge in his voice

[meliss]

Hi. What does it mean "an edge in his voice"?
I'm reading a tale about Wallenberg. "The next day he returned home with an escort to pick up his belongings. Friends described him as calm but with an edge in his voice, assuring them he would be back in about a week. Wallenberg’s friends and family never saw him again."
Thank you.

 

[Skrej]

You could tell from his voice that he was upset.

 

[teechar]

It means a little nervousness in his voice.