Pretty easy - προϊέναι derives from is just the broadened word for "going", so pro- means going towards, going forth ... hence, here, in the debate about the Holy Spirit it means the "going out" of the Holy Spirit, out, not in the sense of leaving the Trinity, but going out of the Father (or as others think, out of Father and Son). ἐκπορεύεσθαι means the same, but takes the opposite perspective. Whereas going out puts agency on the Spirit, the medial or passive form of ἐκπορεύεσθαι looks at the Father (or Father and Son) and this form looks at the Spirit as the one who is moving ἐκ = out of. It is just like looking at the same thing from two directions. pro meaning forth, ek meaning out of.