Generally H/s and Sol/s are used as the same. What you care about is Sol/s.

The mining in ZCash uses Equihash. On each Equihash *iteration* a number of *solutions* are found. This is a randomized process though. Sometimes 0 solutions are found, most of time 1, 2, 3 but it could also be more. On average about 1.877 solutions *can* be found per Equihash iteration. Some miners might though do some short cuts which improve iterations per seconds but do not find all solutions per iterations, hence finding less than 1.877 solutions per Equihash iteration.

Optiminer shows both Iterations/s and Solutions/s. The profit you get from mining is proportional to the Sol/s, therefore that’s what you should care about. The reason why optiminer shows both is that Iterations/s is more stable than Sol/s (and you can compute the average Sol/s as I/s*1.877) . Sol/s has a high variance due to the randomness in the process.