A conditioner or a mask works mostly on the outer layer of the hair — the cuticle. It smooths the surface, makes the hair feel softer and helps with detangling, but it does not change what is happening inside the hair fibre.
A filler is different: it is a low-molecular formula designed to penetrate through the damaged, lifted cuticle into the cortex, which is the inner core of the hair. There it physically fills the gaps that appear after colouring, bleaching, perming, heat styling or long sun exposure. It does this with hydrolysed proteins, conditioning agents and moisture-binding substances, which is why the product is described as an intensive reconstructor — a step between a mask and a styling product.
In practice, this means the filler adds substance and elasticity from the inside, while a mask mainly gives surface smoothness. A typical routine would be: wash with a gentle shampoo, use a nourishing mask once a week, then apply the filler to clean, towel-dried hair before drying or styling. The filler is leave-in, so you do not rinse it out — it stays on the hair and creates a thin protective film that reduces friction during combing and blow-drying.