A sliver is a thin feature, often a few mils wide, left by overlapping pours, clearances or mask openings: a copper wedge between two pads, or a strip of soldermask between closely spaced pads. Copper slivers can lift during etching and land elsewhere as shorts, while mask slivers flake off in processing. DFM checks flag slivers below a set width, and designers fix them by adjusting pours or merging mask openings.
There's a 2 mil copper sliver between those pour edges, it'll flake off and short something, clean it up.
Related terms
Acid trap
Acute inside angle in copper artwork where etchant can pool and over-etch, a classic DFM flag.
Copper pour
Area of copper flooded in by the CAD tool around traces and pads, usually tied to ground or a power net.
Soldermask
Thin polymer coating over a board's copper that leaves pads exposed and keeps solder where it belongs.
DFM
Design for manufacturability: checking and adjusting a design so it can be built reliably at sensible cost.