Yay, youve got an investment portfolio! Look at you. Now, who manages it? Likely a portfolio manager, which can be one person or a group of people at an investment firm. Theyll be the person or people managing the day-to-day stuff regarding your portfolio, including the trading.
Example: Because you probably dont know much about equity trading strategies (why would you?), your portfolio manager does the work for you. Based on the assets in your portfolio, theyll look at the best investments for your goals and buy and sell accordingly for as long as theyre managing your account.