When I do basic option trading, I been just buying calls or puts on stocks I don't own. I just been using 'buy to open' to open a postion and then 'sell to close' to close out, but what is 'sell to open' and 'buy to close'?
Sell to open basically means you are openeing a position buy selling option contracts, whether it be calls or puts. You can short the calls (take a bearish position) or short the puts (take a bullish position). If you already own the underlying equity, selling the calls will equate to a "covered call" strategy where the amount you can short equals the number of shares you have.
Otherwise, selling a naked call or puts is highly risky and not recommended to beginning option traders. Losses are unlimited and margin calls can come quickly, plus most brokerages require high minimum and good knowledge of options before allowing you to make "sell to open" and "buy to close."
Those terms can also be used with option strategies like condors, butterflies, basic debit or credit spreads. I highly recommend you read the "Characteristics and Risks of Standardize Options" booklet provided by your option broker before investing in stock options.