A juxtamedullary nephron is a type of nephron, found only in birds and mammals and not in other vertebrates. The "medullary" is referring to the renal medulla, while the "juxta", (Latin: near), refers to the relative position of the proximal convoluted tubule. In other words, a "juxtamedullary nephron" is one where the proximal convoluted tubule and its associated loop of Henle occur at a deep position compared to most other nephrons.
This type of nephron is relatively rare, and only comprise 20-30% of the nephrons in the human kidney. However, in most depictions of the nephron, it is a juxtamedullary nephron which is depicted. The greater gradient in the deep medulla make this type of nephron do more "work" than more shallow nephrons, and their vertical nature makes them easier to illustrate. It is these nephrons which are responsible for the development of the osmotic gradients in the renal medulla, which are used to concentrate urine.