This is a typical item from the free Basic English collection at SuperMemo.net, which is supposed to teach elementary English vocabulary to beginners. Unfortunately, I lack the wordpower to describe the stupidity of expecting someone who doesn’t know the word door to understand that definition.
To be fair, SuperMemo does offer the same collection in several language versions (for a monthly subscription fee), in which the complicated English definitions are replaced with Polish, German, etc. translations. Still, the purpose of providing a free version that is clearly unusable is something I can’t figure out.