Why is there a brown line of discoloration on/in some of my nails? from bottom to top

Could be many things: Lines of pigment in the nails are often seen in individuals who have darker pigmentation, such as african american or darker latin individuals. Discoloration can also be due to trauma, or in less frequent cases, from a form of skin cancer called melanoma. I suggest you have a dermatologist (if on hands or feet) or podiatrist (if on the foot), take a look and evaluate it to rule out melanoma.