In the first sentence , it's in the active form (grammatically ) that's means that we can identify the object in this sentence .As we know that the rule has been explained by someone and we could understand from (we ) in the sentence.
Subject : you
Object : we
don't you (subject) understand The rule we (object) have just explained.
In the second sentence, it's in the passive form . we don't have an object which means we don't know who explained the rule .
Don't you ( subject ) undеrstand thе rulе (whіch has bееn just еxplaіnеd )
There is another way to the sentence :
Don't you undеrstand thе rulе whіch has bееn just еxplaіnеd (by us )
( us ) here indicate the object because we knew who explained the rule so it will be in the active form again.
I hope this could help