This evening, I opened my wife’s wardrobe and discovered this inside.

The item in the photo is a plastic hollow-wall anchor (also called a hollow-wall plug, self-drilling drywall anchor, or molly-type plastic anchor). It’s a common fastener designed to give screws a secure hold in drywall, hollow plaster walls, or thin paneling where a wood stud or solid backing is not available.

How it’s made and how it works

Shape: Typically a hollow plastic cylinder with ribs or fins and a flanged head; many designs include a center sleeve and a spiraled or ribbed shaft.
Principle: The anchor is inserted into a predrilled or self-drilled hole in the wall. When a screw is driven into the anchor, the anchor either expands or the screw cuts into the plastic and the ribs bite into the drywall. This expansion or gripping action distributes the load over a larger area than the screw alone, preventing the screw from pulling out of the hollow wall.
Common uses

Hanging pictures, mirrors, clocks, and small shelves.
Mounting light fixtures, curtain rods, towel bars, or decorative hooks in drywall.
Securing lightweight to medium-weight items where studs aren’t available.
How to use one (basic steps)

Select the right anchor size for the screw and the weight you plan to hang. Packaging usually lists weight limits and recommended screw sizes.
If required, drill a pilot hole the size specified by the anchor. Many self-drilling anchors can be screwed directly into drywall with a screwdriver without a pilot hole.
Push or screw the anchor into the wall until the flange sits flush.
Insert the screw and tighten until snug. The anchor will expand or grip the wall as the screw advances. Do not overtighten — that can strip the anchor or damage the drywall.
Tips and cautions

Weight limits: Plastic anchors are best for lightweight to medium loads. For heavy items (large mirrors, heavy shelving, TV mounts), use toggle bolts, metal anchors, or secure into a stud.
Choose the right type: There are many anchor styles — plastic expansion anchors, self-drilling anchors, metal molly bolts, and toggle bolts. Match the anchor to the wall type and load.
Avoid overtightening: Doing so can crack the anchor or pull it through the wall.
Remove carefully: If you need to remove the anchor, back out the screw first. Some anchors can be twisted out; others may need to be cut flush and patched.
Conclusion
That plastic piece is a simple, inexpensive wall anchor used to give screws something to bite into in hollow walls. It’s handy for hanging many everyday items, but pick the correct style and size for the load and wall material to ensure a secure installation.