contracts are such that terms are written and then sealed.
if the contract is violated by either party, the seal is broken and the terms of what the consequences are then get revealed.
the seal being broken means that the stipulations of violating the contract are now going to be learned, and the violation will result in some mitigation, as described by whatever was under seal, and now can be revealed.
in a law case it might mean that evidence that had been kept 'under seal' is now to be released to a grand jury, and charges and consequence will follow for those who violated the terms of whatever had been agreed upon.
so it could mean a bunch of things.
I think in the case of what President Trump truthed (on Truth Social) it was about some people who had made agreements to keep the evidence of crimes under seal. But they have violated their agreements and now the evidence is unsealed.
I don't know this for sure.