The kilt pin is a piece of jewellery

Jewellery

Jewellery or jewelry is a form of personal adornment, such as brooches, rings, necklaces, earrings, and bracelets.With some exceptions, such as medical alert bracelets or military dog tags, jewellery normally differs from other items of personal adornment in that it has no other purpose than to...

 that is usually worn on the lower corner of the outer apron of a kilt

Kilt

The kilt is a knee-length garment with pleats at the rear, originating in the traditional dress of men and boys in the Scottish Highlands of the 16th century. Since the 19th century it has become associated with the wider culture of Scotland in general, or with Celtic heritage even more broadly...

. Its function is to prevent the apron falling or blowing open, by adding weight to the outer apron. It does not pin the outer apron to the inner fabric.

It is common for kilt pins to be decorative with many featuring clan

Clan

A clan is a group of people united by actual or perceived kinship and descent. Even if lineage details are unknown, clan members may be organized around a founding member or apical ancestor. The kinship-based bonds may be symbolical, whereby the clan shares a "stipulated" common ancestor that is a...

symbols, national emblems or native animals and plants.