This is the 5448th most frequent Korean word.
마련이다
"It is bound to be" or "It is natural."
Here, 마련이다 conveys that being well-prepared is a natural expectation in this context.
새로운 도전을 하려면 준비를 철저히 하기 마련이다.
When taking on a new challenge, thorough preparation is assumed.
It is used to indicate that the desire for love is a common and logical sentiment.
사람은 누구나 사랑받고 싶기 마련이다.
Everyone naturally wants to be loved.
마련이다 is used to express the inevitability of something occurring.