Murphy's Law is just a famous satirical statement commonly stated as : "Anything that can go wrong will go wrong".

Example on how Murphy's Law work: 'Going abroad for a vacation'

One can prepare for days for his/her flight and vacation but according to Murphy's Law, anything that can go wrong will go wrong.

The day of your flight, you might have overslept, had some car engine troubles while going to the airport, being held inside the airport, weather problems, etc... No matter how much you prepared, if there is even a possibility of 1% that a bad thing will happen, it will happen. That is Murphy's Law!

Obviously this satirical law is not always true. But it has been used often to describe a scenario in which one comes in fully prepared but still failed due to something that does not always happen.

And no! It is not a law in physics, it's mostly associated with the 2nd law of thermodynamics but it have been disproven by many.