Thanks for your question.
Any gains from selling stock are taxed at your ordinary income tax rates if you held the stock less than 1 year or at the long-term capital gains tax rates if you held the stock more than 1 year. Yes, any gains that are made liquid such as when selling stock and moving the money into a money market fund are taxable (unless they are held within an IRA account).
You are only taxed once on any gain. For example, you bought stock for $100 but sold it for $200. The $100 gain is taxable. You later invest your $200 proceeds into another stock. You sell this new stock for $250. You now have a $50 taxable gain to report. Your $100 grew to $250 but you've only been taxed on the $150 total gains.
Hope this helps,