An ink trap is a feature meant to aid printing at small sizes. The right and acute angles are opened up on letterforms. When the type is printed, ink naturally bleeds into the empty area, thus alleviating ink blotting. On glyphs with no ink traps the ink would bleed excessively.
These are purpose-built typefaces for designs like news print, classified ads or other publications with small, dense typesetting.
Lately, designers have been setting type with ink traps at larger sizes, thus emphasizing the ink traps as a design element. They may be doing this not knowing what an ink trap is. It may be trendy, but Al says it’s just wrong. 😡