ChatGPT explains it better than I could ever do :)
Explanation:
LayerToLayerY(...)
Converts the Y coordinate of the OnscreenMouse sprite from its current layer to the coordinate system of the SmallScreen layer. This gives you the mouse's vertical position as if it were on the SmallScreen layer.
unlerp(SmallScreen.BBoxTop, SmallScreen.BBoxBottom, <converted Y>)
Calculates how far vertically the converted mouse Y-position is within the bounds of the SmallScreen sprite.
If the mouse is at the top of the sprite, this returns ~0.
If it's at the bottom, this returns ~1.
If it's halfway down, this returns ~0.5.
So the result is a normalized vertical position from 0 (top) to 1 (bottom), relative to SmallScreen.
lerp(SmallScreen2.BBoxRight, SmallScreen2.BBoxLeft, <normalized Y>)
Now this takes that normalized vertical position and maps it horizontally within the SmallScreen2 sprite — specifically, from its right edge (0) to its left edge (1).
This is effectively flipping the Y-position into an X-position, which may be used for something like a sideways scrolling effect, or rotating the screen logic 90 degrees.
Summary in Plain English:
This expression finds how far down the mouse is on the SmallScreen sprite (top to bottom as 0 to 1), then uses that value to pick a corresponding horizontal position from right to left across the SmallScreen2 sprite.
You could say:
“This calculates the vertical position of the mouse relative to SmallScreen, then maps that position to a horizontal coordinate across SmallScreen2, from its right edge to its left edge.”