Ligatures [0] are special glyphs which replace a series of letters. For instance, an "fl" is often replaced by a single glyph which looks far better than the individual glyphs.

Scunthorpe Sans has ligatures defined for each nasty series of characters, but instead of replacing them with something more readable it replaces them with a black box.

[0] https://en.wikipedia.org/wiki/Ligature_(writing)

Right on the home page...

> How this s** works: Modern fonts can combine letters into a single ligature, usually for things like fi or fl but you can pick anything so we’ve done it for swears.

They are abusing Font Ligatures (https://fonts.google.com/knowledge/glossary/ligature), a feature that allows you to substitute one glyph in the place of two or more other glyphs combined if it looks more aesthetically pleasing. Also used for some other script features / rules.