A brass fastener, brad, or split pin is a stationery

Stationery

Stationery has historically meant a wide gamut of materials: paper and office supplies, writing implements, greeting cards, glue, pencil case etc.-History of stationery:...

 item used for securing multiple sheets of paper

Paper

Paper is a thin material mainly used for writing upon, printing upon, drawing or for packaging. It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets....

 together.

The fastener is inserted into punched holes in the stack of paper and the leaves, or tines, of the legs are separated and bent over to secure the paper. This holds the pin in place and the sheets of paper together. For few sheets of paper, holes can be made using the sharp end of the fastener.
A split pin may be used in place of staple

Staple (fastener)

A staple is a type of two-pronged fastener, usually metal, used for joining or binding materials together. Large staples might be used with a hammer or staple gun for masonry, roofing, corrugated boxes and other heavy-duty uses...

s, but is more commonly used in situations where rotation around the joint is desirable. This lends split pins to use in mobile paper and cardboard

Paperboard

Paperboard is a thick paper based material. While there is no rigid differentiation between paper and paperboard, paperboard is generally thicker than paper. According to ISO standards, paperboard is a paper with a basis weight above 224 g/m2, but there are exceptions. Paperboard can be single...

 models.
It is shaped somewhat like a nail with a round head and flat, split length. Brass fasteners are made of a soft metal

Metal

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

 such as brass

Brass

Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...

 and the tines are typically of two slightly different lengths. This allows easy separation of the tines.
A brass fastener is similar in design and function to the mechanical counterpart cotter pin

Cotter pin

A split pin, also known in U.S. usage as a cotter pin or cotter key, is a metal fastener with two tines that are bent during installation, similar to a staple or rivet. Typically made of thick wire with a half-circular cross section, split pins come in multiple sizes and types.The British...

s.

Brass fasteners are often used as modern scrapbooking

Scrapbooking

Scrapbooking is a method for preserving personal and family history in the form of a scrapbook. Typical memorabilia include photographs, printed media, and artwork. Scrapbook albums are often decorated and frequently contain extensive journaling...

 embellishments and in Duo-Tang brand paper folders.