Over 700 finance terms, Shmooped to perfection.
Active management of money. You buy shares in a mutual fund, which has hired theoretically smart analysts and portfolio managers who spend all day looking at stocks and bonds and make investments in them or sell them on your behalf. For a small fee. The other side of the street is passive investing which is just buying an index fund, which doesn't really have a portfolio manager per se - more like a rebalancer who just makes certain each period (usually quarterly) that the index still reflects what you think you bought in the first place.