As an adjective, covert
means secret, concealed, or disguised — think of a spy who is undercover
. James Bond, for example, often performs covert operations in which he sneaks in, gets the information he needs, and sneaks out unnoticed.
If you turn covert into an adverb and do something covertly, it means you do it secretly, surreptitiously, or stealthily, as in this excerpt from Mark Twain's The Prince and the Pauper:
During two or three days, Hugo, in whose ward and charge the King was, did what he covertly could to make the boy uncomfortable; and at night, during the customary orgies, he amused the company by putting small indignities upon him — always as if by accident.
The opposite of covert is overt, referring to something evident or obvious.
Covert can also be used as a noun to refer to a covered or protected place or a hiding place, as illustrated in this excerpt from Edith Wharton's Ethan Frome:
[Mattie] pronounced the word married as if her voice caressed it. It seemed a rustling covert leading to enchanted glades. A pang shot through Ethan, and he said, twisting away from her in his chair: "It'll be your turn next, I wouldn't wonder."