"I've been doing optimization for our websites and tracking the results regularly but don't really know what the results actually mean."
This is quite a broad statement. I'll take it that you already understand the important parts of the optimization process. So, the first question to address would be: What results were you trying to achieve?
If all you're going for is "more traffic," then you can simply look at Analytics and see whether or not organic traffic has noticeably risen or fallen to get a measure of your success. If your goal is more business, then you need to start looking at conversions and specific page flow within Analytics.
For instance, say you run an e-commerce site... perhaps you should look at the number of hits to the "thank you" page that users see after making a purchase. Is it getting more hits than it has in the past, and if so, does the increase in hits there correlate with your increased traffic, and does all of that together correlate with your SEO efforts? In a perfect world, things would look like this:
- Optimize your site
- Traffic noticeably increases
- Conversions noticeably increase
I'm sorry if this is all a little broad... it might help if we know exactly what "results" you'd like to track.