Fruit turns brown when damaged or rotting because chemical compounds in the fruit (called phenolic compounds) are oxidized (interact with oxygen). It is easier for oxidation to occur when the fruit is damaged, because the fruit cell walls and membranes break, allowing oxygen in. When the cell´s compounds react with oxygen, they tend to turn brown. If you coat acid (e.g. lemon juice) on a broken fruit (e.g. a sliced avocado or apples) this will slow down the browning (oxidation) because acids react preferentially with oxygen, thereby reducing how quickly oxygen reacts with the fruit compounds.