It seems like what the first equation is saying is - the return on the stochastic discount factor is equal to the zero-mean innovation to the pricing kernel discounted at the riskless rate.
I believe a good analogy to understand it is - the change/evolution in the stochastic discount factor throughout time can be attributed to a riskless $\frac{1}{1+r_f}$$1+r_f$ and a random $1+e^M_{t+1}$ component, which at least to me, makes a lot of sense.