This action block sets the image_alpha value for the instance. The image alpha is the "transparency" value for drawing any sprite assigned to the instance and has a value between 0 and 1, where 0 would be fully transparent and 1 would be fully opaque. Note that if the sprite has transparent or semi-transparent pixels, then their transparency will be scaled by the amount, so if you have some 50% transparent pixels in the sprite and set the instance alpha to 0.5, the final alpha for the transparent pixels would be 25%. You can set this value to be a relative amount
Please note that for changes in this variable to be visible, the instance should have either no draw event (and so GameMaker Studio 2 will default draw the sprite) or be drawn using Draw Self action.
Argument Description alpha The alpha value to set (from 0 to 1, default is 1).
The above action block code sets a new sprite as well as a number of other properties for how that sprite is to be displayed, including setting the image alpha to be 0.5.