The randdom node generates numbers between 0 and 1. I’m saying if that number is > than .25, make then cube. In other words cubes have a 75% chance of begin drawn.
You can say, if the random number is between 0 and .3 ( for instance ), then spawn a glass cube, otherwise spawn a normal one.
Or you could split the 0-1 range into 3 and draw 3 different types of cube.