What does 100% black font mean?

In CMYK, the K means black. When you convert RGB black to CMYK black, the black is actually a combination of all 4 colors. To get the black font to be true black, you will need to reduce the C, M, and Y colors to 0% and the K to 100%. This should be done for all true black fonts to prevent the printers from going over your text four times with all the colors.