There are so many private keyword tools out there and each has its own way of presenting the information. If that's the case, you need to look at the particular resource you've selected.
Normally, "Rank" relates to the positioning.
"Common Terms" could be that tool's assessment of the generic phrases.
"Highest Rank" is what it says, the highest ranked term, perhaps on the site assessed.
"Top Ranked" is probably as above, but perhaps the keywords you've chosen. Difficult to say.
One of the best ways to learn is to do it yourself without using proprietary tools.
A good starting point is:
1. Check out your competitors' websites and look at their keywords (don't copy them, you can do better).
2. Run a few keyword checks in your favourite search engines to detect which sites do well.
3. Analyse those sites to find out why.
4. Use a spreadsheet to build a record of keywords, and their extent.
5. Once you've done that, get yourself a thesaurus and start adding keywords to your spreadsheet.
I don't know i've answered your question, but hopefully, it'll help you learn by minimising the proprietary tools you use.
Mod note: please, no tool drops.