Leca is an acronym and it stands for: Lightweight expanded clay aggregate. It sounds very complicated, but it's quite easy to explain. Essentially, Leca is a growing medium, like soil, in which you can grow your plants. Leca is a collection of baked clay balls that expand when you soak them in water.
This is a companion discussion topic for the original entry at https://plantcareforbeginners.com/articles/what-is-leca-why-useful-for-growing-plants