When you sell a call to open a covered call, you create a Short Position option position. A buy-to-close order is used to exit that short call before expiration by repurchasing the same contract.
Traders often buy to close after the option’s premium has decayed. If the call you sold for $1 falls to $0.10, buying to close locks in most of the premium and frees the shares to sell a new call. It also limits losses when the trade moves against you.
Buying to close leaves the underlying stock in your account. You’re then free to Sell to Open another call or simply hold the shares.