Leca is an acronym and it stands for: Lightweight expanded clay aggregate. Essentially, Leca is a growing medium, like soil, in which you can grow your plants.
In simple terms, Leca is a collection of baked clay balls that expand when you soak them in water. The water that's soaked up in the clay balls can be used to water plants. However, unlike soil, Leca is just a collection of clay balls and doesn't contain any nutrients. All they do is soak up a little water and expand. Your plant will still need nutrients, so you'll need to add these nutrients to the water.
