The double curly quotes are standing in for double primes which indicate inches. See:
http://www.shapeoko.com/wiki/index.php/Designing_for_Fabrication#Characters
For those who are curious, the image in question came from: http://docs.carbide3d.com/article/16-nomad883-materials or http://carbide3d.com/nomad/feedandspeed/
There’s also some discussion at: