A fund manager, as the name suggests, is the person who manages your fund. They take the pool of money in your fund and invest that on your behalf with the aim of meeting the specific objectives of that fund. They will have to stay within the guidelines set out for that fund in terms of how much risk they can take and the types of assets they invest in.
They are professional investment experts - it’s their job to try and invest in the best performing assets to get the best return for you. They usually have the help of large research teams who track asset performance to help identify good investment opportunities.
Their services don’t come for free and charges should be something you look into before choosing a fund, but for most investors their expertise will prove invaluable.