The concept of isocost
In the use of resources, firms are faced with opportunity cost. For every addition of say capital, they must forego a unit of say labour.
Suppose the firm pays:
W for L (labour)
r for k (capital)
If C is the Total Cost (TC), then
C = rk + WL. So if C is given as c, then the producer can choose among various combinations e.g.
k = C - WL
Thus if he spends all the money on k then he shall be at A and if he spends all the money on L then he shall be at B. At A he spends C while at B he shall also spend C. The line joining A and B is called Isocost line and is defined as locus of all different combinations of factors the firm can purchase given a stipulated money outlay and factor prices.