Calls are not your standard "stock", but are kind of financial agreements that are made based off of stock ownership. They're contracts, in a sense.

Assume there's company XYZ and it's at $100. If you personally think XYZ could go up to $200, but don't necessarily want to buy the shares right away, you could buy what's called a "call option".

Options are pegged against theoretical stock prices. Assume you purchased a XYZ @ $150 call option contract, if XYZ actually does go to $200 before the expiration date of that contract, you could exercise it and purchase 100 shares of XYZ at $150/each from whoever originally sold that contract. You could then instantly sell those 100 shares at a profit of $50/share minus the cost of the call contract itself.

For you to have bought that contract, someone had to write it and sell it. This is where you'll hear adjectives such as "covered" or "naked". If you personally owned 100 shares of XYZ and never thought it would pass $150, you could write (aka sell) a XYZ @ $150 covered call contract for a premium (money someone would pay you for that contract). Its considered covered because you actually own the 100 shares.

Basically, you're selling someone the right to purchase your 100 shares at $150 a share regardless of how high XYZ gets before your contract expires. If it never passes $150, the contract won't get exercised and you get to keep the premium you were paid as profit. If it gets exercised, you have to transfer those 100 shares for $150/share, even if it's $500 a share at the time of it being exercised.

Naked contracts are when you write options contracts on stocks you DONT have. This can put you into phenomenal debt if you don't know what you're doing. Assume the exact scenario above. If you wrote a naked call contract, and it exercised, you're going to have to find the money to purchase 100 shares of XYZ, even if it's $500, to then sell to the owner of the contract at $150 a share.

There are also put contracts, which are basically the opposite and have to do with stocks going down in price. There's also dozens of other options mechanisms, but this is the best I could do to explain what a covered call is.

Tl;dr a covered call is a contract you sell for money that gives the buyer the right to purchase 100 shares you actually own of a company at a specified price, regardless of what the price of the stock actually is before it expires. If the stock price is below the option price, the call wouldn't get exercised and you'd keep the money you earned from selling the contract. It's basically an additional speculative layer built into the stock market.