Final answer:
The tracks on either side of the nail that guide its growth are called the nail folds, which are folds of skin anchoring the nail plate.
Step-by-step explanation:
The tracks on either side of the nail that the nail moves on as it grows are called the nail folds. These are folds of epithelium at the sides of the nail body, holding it in place. The nail itself originates from the nail matrix at the proximal end of the nail. As stem cells in the nail matrix divide, the newly formed cells create the nail plate or nail body, which is composed of densely packed dead keratinocytes. As the nail grows, the edges of the nail plate are embedded in the nail folds, which help to anchor the nail in place and protect the nail margins.