Assuming it's translated literally from French, and the meaning of the French verb "couper" hasn't changed since the Middle Ages, it means mixing with water, and is a widely used expression in French.

I’ve always heard it generally used as a synonym with dilute - generally speaking with drugs, cut often means incorporating any less expensive (not easily detectable) substance in with the original substance.

More times than not it will not be stronger, it will be compounds that are not psychoactive at all, effectively “cutting” the potency of the substance while multiplying the quantity.

There are cases with fentanyl where a stronger substance is mixed in with the original and this often is what you read about in the news, but it is not in generally in the distributors best interest to be killing their clientele.