Please tell me what does a palate expander really do?

Expands the palate: It really does expand the palate! the name they came up for it perfectly describes what it does. In essence, it moves bones of the maxilla that have not yet fused, thereby expanding the palate.

Palatal expansion: Before the suture line in the palate fuses together, a palatal expander exerts force to move both sections of the palatal bone further apart, expanding the palate and thereby moving the upper right and upper left teeth with it in order to create a larger arch form.