Initially as you add more of a factor of production, the marginal product will increase. This is because each unit of input added will add more ouput than the one before (because of specialisation and division of labour). Eventually, if you keep adding units, the fixed factor of production will begin to limit the additional output. (For example, if there is only 1 large machine with 3 buttons, the 4th worker added will provide less output than the previous 3 workers because there is no space or capital for him).