This labelFont setting basically works as the CSS font-family property. You first need to load the font the way you want (using @font-face for instance). Then, you set labelFont with the name of the variable, with fallbacks separated with commas if needed (like Lato, sans-serif for instance).

To be a bit more precise, sigma assembles its settings labelFont, labelSize and labelWeight into the Canvas font property. The code is visible here.