If you, or your attorney, has filed a claim for personal injuries stemming from your accident, the insurance company may send you a "reservation of rights" letter. What that is is the insurance company protecting themselves. It is their way of saying that just because we have acknowledged your personal injury claim, we have not agreed to pay out on the claim. It is meant to tell the claimant (you) that they have not decided if the claim is covered under the relevant insurance policy, and that they are in the process of deciding if they have to pay the claim.
The bottom line: don’t worry if you receive such a letter. It is simply a way of preventing you from later claiming that the insurance company has to pay on your accident claim simply because they acknowledged the claim.