As already mentioned, it is from a tellers stamp. When the notes are bundled into stacks of 100 or whatever amount that value note gets, they put a paper strap or band around it to hold it together, and then stamp it with their "Tellers Stamp" to show who did it. The stamp often goes off of the band and ends up on the note. NAV, no added value and in fact it ruins a collectors note value.
Spend it. You'd have a tough time selling a circulated star note like yours for a premium above face value.
When the notes are in their bundles of 100, a paper strap is used to keep them together. A common practice is to stamp the strap to show it's been counted and this looks like an over ink on the strap, as @tommyc03 suggested.
When a bank teller is counting bills they then wrap them with a paper strap. Then initials are placed on the strap and the strap is dated with a teller's stamp. Sometimes that ink gets on the bill and that's what happened to this note. At the end of the serial number is a star, meaning this bill was a replacement note. Nice, but common.