Covert means secret or hidden. Soldiers might take part in a covert mission to infiltrate an enemy camp — and you might take part in a covert mission to steal your brother's leftover Halloween candy.
Covert is the opposite of overt, which means obvious, or in full view. Covert things are hidden, private, or stealthy. It comes from the Old French covert, or "covered." A covert bag of chips you've snuck into the movie theater might literally be covered, hidden under your coat, while a covert plan to throw your dad a surprise party is just a secret.