Course Accessibility Report
Access to the Course Accessibility Report is through a widget on the homepage of your D2L course. It may be important to note that this widget is only available to the instructors of the course. (Students, course reviewers, etc., cannot see this widget. It is only available to instructor roles.)

Course Accessibility Score
Here is an image of the Course Accessibility Score for this course:

Across the top, you can see that my accessibility score is at 96%, the gas meter is green and almost full, indicating a higher score.
All Content
Below that, another chart on the left side, showing that I have 201 documents in the course with a breakdown of the number of files by type, 148 files are HTML, 23 files are simple descriptions (think topic descriptions), 13 Modules (which I only have 12 root modules, I don't know where the extra one comes from), 11 Word documents, and 6 PDF documents.
This chart also comes with a button, View. On clicking the button, I am taken to the content tab (which is between the Course Score and All Content chart).
From this view, I am able to see individual content scores:

You can see I have a very low score on some of these pdf documents. When I click on the first document, I get to see steps toward remediation of the document.

You can see Ally suggests fixing the untagged nature of the document first. Ally explains what this means (a button) and then provides instructions on how to tag a PDF (another button).
Fixing the document
To do this efficiently, because this is a PDF, you will have to download the document and fix it in Adobe Acrobat, following the directions Ally gives you. (If you need Adobe Acrobat, I suggest you utilize https://labs.metrostate.edu which is free to you and your students--a fantastic resource brought to you by ITS.)
When you have completed the document, upload the document again by dropping it in the upload spot (or using the Browse button to browse for the file) that Ally provides.
It is important for us, that you utilize this workflow. We want to be able to say, here are all the documents that have been improved upon, showing the value of the tool.
I mentioned the download button on a PDF. Word documents do not have a download button. To get to the Word document download, click on the button that would explain how you would remediate it. There will be a download link on that screen.
See all issues for the document
If you click on the All issues button to the right of the meter, Ally will show what is wrong with the document and it will indicate how to raise the score:

Easy Fixes
Ally also provides, back to the report page, an ability to jump at low hanging fruit with their "Content with the easiest issues to fix."
In the example, upon clicking on start, I was shown two documents that are easy to fix:

When I click on the document, Ally pops up the remediation window and explains that the document has two images without description. I can quickly download the document and upload the fix through Ally.
Low Scoring Content
If you are looking to improve your accessibility, perhaps you will want to start with the documents that are not accessible. That is the point of Ally's low scoring content button. Clicking on the button, I am met with my 5 lowest scoring documents that I need to work on. Clicking again on a specific document, I am then met with steps for remediation.
Resources
Blackboard Ally provides good documentation on Course Accessibility Report opens in new window, check it out.