What is dissenting opinion in a supreme court case

It's the opinion of a justice who disagrees with the ruling that was reached by the majority of justices.

I assume OP refers to the United States Supreme Court or a supreme court (highest appellate court) in one of the several states. I note that in New York, and perhaps a few other states, a "supreme court" is a trial court, and not an appellate court.

A dissenting opinion would be written by one of several justices on such an appellate court, which justice would be in the minority in regards to the decision.

A dissenting opinion is generally not precedent (i.e.: binding law on the issues discussed), but can sometimes be cited in other cases and argued to be "persuasive".