If it is on the right side of the function it means that it is one of the independent variables that explains the dependent variable (located on the left side of the function). What the function is representing is the influence that the independent variable (the cause of the dependent variable) has on the dependent variable (the effect that the cause or causes are being sought).
The expected rate of return of a stock is the mean return that is expected to be earned by the stock considering the different scenarios that can occur, the return in these scenarios and the probability of the occurrence of these scenarios. The formula for expected rate of return of stock is,
rE = pA * rA + pB * rB + ... + pN * rN
Where,
pA, pB, ... represents the probability that scenario A, B and so on will occur or the probability of each scenario
rA, rB, ... represents the return in scenario A, B and so on