A seam ripper is a small tool

Tool

A tool is a device that can be used to produce an item or achieve a task, but that is not consumed in the process. Informally the word is also used to describe a procedure or process with a specific purpose. Tools that are used in particular fields or activities may have different designations such...

 used for unpicking stitches

Stitch (textile arts)

In the textile arts, a stitch is a single turn or loop of thread or yarn. Stitches are the fundamental elements of sewing, knitting, embroidery, crochet, and needle lace-making, whether by hand or machine...

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The most common form consists of a handle, shaft and head. The head is usually forked with one side of the fork flattening out and becoming a blade and the other side forming a small point. In some designs the blade side then tapers back to a point to allow easier insertion in tight stitching.

In use the blade is inserted into the seam

Seam

Seam may refer to:* Seam , the line where two or more layers of fabric are held together by stitches* Seam , a stratum of coal or mineral that is economically viable; a bed or a distinct layer of vein of rock in other layers of rock...

 underneath the thread

Yarn

Yarn is a long continuous length of interlocked fibres, suitable for use in the production of textiles, sewing, crocheting, knitting, weaving, embroidery and ropemaking. Thread is a type of yarn intended for sewing by hand or machine. Modern manufactured sewing threads may be finished with wax or...

to be cut. The thread is allowed to slip down into the fork and the tool is then lifted upwards allowing the blade to cut through the thread. Once the seam has been undone in this way the loose ends can be removed and the seam resewn.