The difference is that they represent different multiples of s, and they differ in terms of their coefficients. They differ by s, and they're both one-term monomials with respect to s, so they're the same kind of terms that you can merge.
The coefficients are 8 and 7, and these two coefficients are even and odd, they are consecutive integers, so they differ by 8-7=1, so the difference between the two monomials is :8s-7s=(8-7) s=s. If s represents time seconds, then s8 is 1 second more than s7.