A 20-minute fire-rated door is one that isolates a room during 20 minutes whilst exposed to fire. This kind of door is known as fire-rated EI2-20, and it is useful for places where you want extra protection even though a fire-rating is not demanded.
Flame-proof doors usually also have some fire-rating. Our Asturmadi Multipurpose E30 Flame-proof door, for example, is also 20 minutes fire-rated. It means it isolates from fire during 20 minutes and holds its integrity during 30 minutes (it would not open or break before the stated 30 minutes of fire-exposure).