3D printed objects can be hollow inside or filled with infill material. This filling material is called infill. The infill is specified as the percentage of the filling material in the total print object.
The infill determines the stability and flexibility of the print object. A more stable structure has a higher percentage of infill material, a more flexible one a lower percentage. If the object is hollow, then it has no infill material and the percentage of infill is 0%. The situation is different for an object that is completely filled with infill. In this case, the percentage of infill is 100%. Such a printed object is naturally more stable than a hollow one.
3D printed objects without infill are called hollow.