It's often used in scientific terms, especially in biology and zoology, to describe things that are thin, narrow, or have a small size.
Here are some examples of words using the root "lepto":
* Leptocephalic: Having a thin head (referring to certain fish larvae)
* Leptospirosis: A bacterial disease characterized by thin, coiled bacteria
* Leptomeninges: The thin membranes that cover the brain and spinal cord
* Leptothorax: A genus of small ants with slender bodies
* Leptotene: A stage in meiosis where chromosomes appear thin and thread-like
Keep in mind that the meaning of "lepto" can sometimes vary slightly depending on the context.