Anything that's sealed is securely — maybe even permanently — closed or unavailable. An ancient sealed tomb may be safe from curious explorers or grave robbers.
A sealed bottle of medicine is tightly shut, not even allowing air inside. A sealed document might be physically closed, like a sealed envelope that's pasted shut, or just unavailable for anyone without special permission to look at. When a promise or vow is sealed, it's officially established or decided: "Their sealed promise meant that neither of them could ever mention the secret club."