Many states require a plaintiff to file this affidavit early in the case to prevent frivolous lawsuits against doctors, engineers, or architects. It serves as a screening mechanism to ensure the case has a valid basis.
In this affidavit, a qualified expert (usually another doctor or professional in the same field) swears that they have reviewed the case records and believe there is a "reasonable probability" that the defendant's conduct fell below the acceptable standard of care. Without this sworn expert opinion, the court may dismiss the case entirely.