Replication of portfolio

In SchweserNotes, there is an example for replicating and risk-neutral pricing:

For a share of stock and a short forward at 50 with six months until settlement, we can write: S − F(50) = 50 / (1 + Rf)0.5 and replicate a long forward position as F(50) = S − 50 / (1 + Rf)0.5

Can anyone explain what do S and F(50) stand for since I am so confuse about these equations.




-S=Short stock

S=Long stock

-F(50)=Short forward

F(50)=Long forward

Specifically, the S pot price of the stock.

Specifically, the current value of the F orward contract with an agreed price of 50 per share.