Here's a corporate trial. To prosecute their case that a competitor stole some of their trade secrets, they have to reveal to the court what those trade secrets are. ("They stole our secret stuff, but we won't tell you what" doesn't fly in court - even SCO got burned eventually for trying it.)
But you don't want that stuff sitting in a court record that anyone can walk in off the street and ask to see. So part of the record get sealed - not available to the public. It's still available to the judge, and to the (outside) counsel for the other side, and to the appeals judge if things go that far.
And only certain details get protected. One side has to ask for it, and the other side can protest, and the judge has to weigh the protection for the side that wants it sealed against the interest of the public to know what went on. More, a redacted version is (usually?) released eventually.
But you don't want that stuff sitting in a court record that anyone can walk in off the street and ask to see. So part of the record get sealed - not available to the public. It's still available to the judge, and to the (outside) counsel for the other side, and to the appeals judge if things go that far.
And only certain details get protected. One side has to ask for it, and the other side can protest, and the judge has to weigh the protection for the side that wants it sealed against the interest of the public to know what went on. More, a redacted version is (usually?) released eventually.