Stealth means to do something so quietly and carefully that no one notices, like the stealth of a kitten sneaking up on a mouse.
Stealth and steal come from the same root word and used to mean the same thing. Stealth is still a certainly a quality useful in stealing, but the meaning has branched out. It can also be something quietly done in secret. For example, a stealth bomber is an aircraft that's hard to detect by radar. Or if you plan a birthday party by stealth, then the guest of honor will be pleasantly surprised.
