The driving skills portion of your CDL training will test your ability to handle the operation of a commercial motor vehicle.
Your skills test will include the pre-trip inspection, road test, and backing tests. You must pass all 3 exams to get your CDL.
The specific types of backing tests will vary by state, so check with your state DMV or licensing bureau.
The skills test is what you've been waiting and working for, and your chance to show that you've learned enough to be trusted with the operation of an 80,000 vehicle.
As part of the exam, you will be expected & required to show that you can properly inspect your vehicle to determine its road-worthiness.
The road test part of the exam will put you behind the wheel on public roads, in an actual driving situation. Normally, you will have had practice in this exact scenario so it won't be totally new to you.
Every state will have different requirements regarding the backing test, and your school will have you prepared for that specific state's rules, but it will generally contain some combination of straight backing, offset backing, alley dock, and parallel parking.
Generally, depending on the state, you'll be allowed a certain number of errors for each section of the CDL skills test.
If you fail any part of the CDL skills test, you will have to re-take the entire exam. Most states will have a minimum waiting period, and limit the re-takes you are allowed within a certain time frame.