Keywords and "Match Words" are used interchangeably. They both refer to the variations that google makes bold in the search results for your search term. When a factor name says "Keywords" it means any combination of these variations. When a factor says "Search Terms" it means exactly the search terms used to create the report.
About the Letter Grades
The letter grades were put in so you would have a way to show a client that there is always room for improvement. They don't actually mean very much in terms of SEO because websites rank relative to the competition. You can rank #1 with a C average if that is better than the competition. This is fairly common and it is rare for websites to have top grades.
Page Size
These are the factors related to how much content is on the page.
Meta Tags
These are the factors related to how much tuning has occured in meta tags.
Title Tags
These are the factors related to how much tuning has occured in title tags.
Headings
These are the factors related to how much tuning has occured in H1-H6 Headings.
Navigation
These are the factors that are often related to tuning navigion. Not always though.
Content Tuning
These are the factors related to how much tuning has occured in the sentence content of the page.
Content Formatting
These are the factors related to the formatting of the content on the page.
Images
These are the factors related to images on the page.
Videos
These are the factors related to videos on the page.