What does a star on a silver certificate mean?

A star on any US bill means that it was issued as a replacement for a bill that was damaged during printing.

The damaged bill (or more often, bills) will be removed from the set and destroyed. The mechanism that prints individual serial numbers makes it difficult to print new bills with the exact same numbers as those that were destroyed. Instead, a special print run is made to replace all damaged bills at the same time. That print run has its own set of serial numbers, but the starting or ending character is a star rather than a letter to indicate that the bills aren't from the original printing.

Because "star notes" are uncommon many collectors will pay a premium for them. The amount varies with the bill's denomination and condition. A worn $1 star note may only bring 25¢ to $1 extra, but an uncirculated $100 star note could sell for significantly more than its face value.