Stealth means sneakiness. When you do something with stealth, you do it so quietly and carefully that no one notices. You might admire the stealth of your cat when she sneaks 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.