OBP is basically batting average augmented by walks. It's hits plus walks divided by plate appearances (with plate appearances being all at-bats and walks). So instead of it being number of times a hitter reaches base per at-bat, it's the number of times per plate appearance. SLG is supposed to measure power. A batter gets 1 for every single, 2 for every double, 3 for every triple, and 4 for every homerun. Divide that total by at-bats. So the answer to your question is yes, .690 means 69/100 of a base per at bat. For OPS I usually just add the two together, though there may be a more precise mathematical formula. In any event my OPS has never been more than .001 different than the one given on a website. AFAIK, there's no way in words to describe it in terms of "90% of something".