A fund manager is an investment expert whose job it is to seek out the best investment opportunities within the particular rules that are imposed on your funds. Taking the example of the European Fund in 'What is a fund?' question, the manager may be restricted to buying shares in EU based companies only. However, the manager will then need to decide:

How much to invest in a particular country eg 25% in France, 15% in Germany, 10% in Italy, etc.

Within each of these countries, how much to invest in each economic sector eg 3% in Banks, 5% in Manufacturing, 4% in Retail, etc.

Within each of these sectors, which particular companies to invest in?

Having decided where to invest the fund, it’s the manager’s job to continually review his/her decisions and to adjust the make-up of their fund as circumstances change.