The sentence says that forgetting things is only natural for any kind of life form including human beings.
불과(不過) : only, just
망각(忘却): oblivion, forgetting, neglecting
인간이 기억할 수 있는 시간은 불과 몇 시간입니다:
The (short term) human recollection capability lasts only a few hours.
망각의 동물: creature that forgets. (and it's natural)