-는 법이다 means "its natural that" or "supposed to be" - expressing norms or expectations.
Natural principle - 세상은 그런 법이에요 (thats just how the world is)
Custom/norm - How things are supposed to be
General truth - Accepted way of things
See examples below for varied real-world usage.
법 = law/rule/way
Similar to -기 마련이다 but softer
Its natural for people to make mistakes.
When winter passes, spring is bound to come.
No matter how much of a secret it is, it is natural that it will eventually be revealed to the world.
If you put in a lot of effort, good results are supposed to happen.
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