What is a filler coin?

Answer:

A filler coin is a low-quality coin that a collector uses as a place-holder in their collection until they can acquire a better specimen.

A lot of collectors pride themselves on having a complete set of all dates and mint marks of a particular series, so filler coins can round out a collection temporarily. A simple example would be a new collector who wants to assemble a complete set of XF Jefferson nickels. Most dates can be found in that condition for a quarter to $5 or so, but 1939-D nickels are rarer and sell for around $10-12. If they didn't want to spend that much on coin for a starter collection, they might settle for a 39-D in VG condition so the set would at least be complete, then later "trade up" for a better coin.