The Transparency property controls the visibility of a part on a scale of 0 to 1, where 0 is completely visible (opaque), and a value of 1 is completely invisible (not rendered at all).
BasePart/Reflectance can reduce the overall transparency of a brick if set to a value close to 1.
While fully transparent parts are not rendered at all, partially transparent objects have some significant rendering costs. Having many translucent parts may slow down the game’s performance.
When transparent parts overlap, render order can act unpredictable - try to keep semi-transparent parts from overlapping to avoid this.
BasePart/LocalTransparencyModifier is a multiplier to Transparency that is only visible to the local client.
This code sample gives the local client X-ray vision using LocalTransparencyModifier. It allows the player to see through all parts in the Workspace, which are found using recursion.
This code sample shows how a part can fade away when touched by a Humanoid then reappear a moment after to create a passable door.