A higher-level review for VA disability means that the claim will be examined and re-adjudicated by a senior rater. The way it works is, the senior rater is not supposed to take any of the previous rating into account and will take the claim from start to finish again to see if there are any errors on the part of the VA or if they believe the evidence submitted is enough to grant approval.
One important thing to note is that you are NOT ALLOWED to submit any new evidence for a higher level review. If you have new evidence that you want to submit for your claim, you would need to file a supplemental claim.
The errors the higher level review look for are errors in the processing of the claim. If the VA did not make a reasonable effort to help you with your claim. This reasonable effort would include things like contacting the NPRC to retrieve your DD214 or retrieve your medical records. Though Service Officers always recommend the Veteran supply those documents themselves, the simple truth is that if the Veteran does not have them, they can still file a claim. The VA, under its duty-to-assist, must make reasonable effort to help the Veteran by asking for the documents from the place most likely to have them, in this case the NPRC.
And remember, if the higher level review denies the claim also, you have one year from the date of the denial letter to file another appeal. Your choices are to get new and relevant evidence to submit in a supplemental claim, or to appeal to the Board of Veterans Appeals.
In any case, it is possible to continue to appeal VA denials for as long as you wish to, and as long as you keep filing them within a year of the previous denial letter, your date of claim will remain the same. This means if you finally get approved, you will be back paid all the way to the original date of claim.