The two dimensions are to open the trade (creating a position) and to buy or sell (becoming long or short the option).
If you already own an option, you bought it to open and then you would sell it to close.
If you don't own an option, you can either buy it to open, or sell it (short it) to open.
If you are already short an option, you can buy it back to close.
If you sell to open covered, the point is you're creating a "covered call" which means you own the stock, and then sell a call. Since you own the stock, the covered call has a lot of the risk of loss removed, though it also subtracts much of the reward possible from your stock.