Can you help me with these economics concepts?

In this case 'X" is the marginal unit (the additional unit of the variable factor). In this case X is netiher before or after as it is just an 'additional unit'. The idea is that at first by adding adding more of the variable input factor, the firm is going to receive increasing 'marginal returns', however it reaches a stage where if any more of the variable unit is added, marginal returns will begin to fall.