Font Size - This will set arbitrary font size. That is if you set smaller font size you'll get a small characters separated form each other. If you select bigger font size your characters could be so big that they will be overlap each other.
Small Font Size (Left picture) v.s. Large Font Size (Right picture)
 

Font Scale - With this control you may select a difference between font size used for light areas and font size used for dark areas of the image.
Low Font Scaling (Left picture) v.s. High Font Scaling (Right picture)
 

Bold Threshold - Areas that are darker than a selected threshold will be filled with characters in bold font face, lighter - with normal font face. Please note that not every one font face has a bold version. If you select a font face that has not bold version this control will not affect the resulting image.

Black Threshold - This control is essential when you using combined colors for font or background. That is colors with names like White or Source color and otter colors with OR in their names. Areas that are darker than a selected threshold will be filled with the first color, lighter - with the second one.

Outline Every Glyph - Stroke every character.
Outline Whole Text - Stroke whole text. This is useful when you use some artistic fonts that mimics handwriting. like a "Script" font. Please see pictures below.

Stroke every character (Left picture) v.s. Stroke whole text (Right picture)
 

Replace Spaces with - You may replace spaces with a character of your choice to avoid visible gaps between words. It may give you an image with more dense color.

Random Glyph Order - characters are drawn in random order. If Random is not enabled - characters are drawn from left to right and from top to the bottom. Please see pictures below.
Ordered Glyphs (Left picture) v.s. Random Glyphs (Right picture)
 

Autolevels - Checkbox - when checked it will improve image quality. White areas will be more white, dark areas - more dark. Despite the fact that this correction may affect real colors of the photo it can be very useful when you making grayscale text images.

Random Rotated Glyphs - This will tilts every character in a text at random angle. Example of Random Rotated Glyphs:

Color #1, 2, 3

You may select three custom colors and use them for Text, Outline or Background.