A more thorough explanation:

Prejudgment interest is the interest that a creditor, usually a plaintiff in a case, is entitled to collect from the amount of a judgment. This interest compensates the creditor for an injury that occurred before the judgment.

The rate of prejudgment interest varies by state, and some states may apply a fixed amount, while others may tie the rate to an established index. For example, the California Constitution applies a general rate of interest at 7% per annum, and in Palomar Grading & Paving, Inc. v. Wells Fargo Bank, N.A., the California Court of Appeals found that this 7% applies to prejudgment interest, at least on a mechanic’s lien.

When a court awards prejudgment interest, it is usually in addition to the damages awarded to the plaintiff. For example, in In re Oil Spill by the Amoco Cadiz Off the Coast of France, the Seventh Circuit awarded French plaintiffs $65 million in damages and $148 million in prejudgment interest in a suit arising out of the grounding of an oil tanker which resulted in a massive oil spill along the coast of Brittany. Another example is Short v. U.S., where the Federal Circuit awarded the Yurok Indians prejudgment interest on their damages to recover revenue generated from timber harvesting from which they were wrongly excluded.

These examples illustrate how prejudgment interest can significantly increase the amount of compensation awarded to a plaintiff. In both cases, the plaintiffs were awarded millions of dollars in damages, but the addition of prejudgment interest nearly doubled the amount they received.