The term "fishtail" can refer to a number of different things, depending on context.
In general usage, it may describe an object or action that resembles a fish's tail - for example, a type of curved knot used in tying shoelaces is sometimes called a fishtail knot because the loops created by the knot resemble a fish's tail.
In more specific terms, "fishtail" can also refer to a particular style of haircut or braid where hair is swept up and back in a curved shape that again resembles a fish's tail. This type of hairstyle has been popular for many years, particularly among women in traditional cultures around the world.
In addition, "fishtail" can also refer to a specific type of rock formation found in certain areas, where layers of sedimentary rocks have been folded and tilted over time to create a curved or wavy appearance that again resembles a fish's tail.