long time ago, windscreen glass are just manufactured just as a piece of glass.
when accidents occur, they will break into pieces big and small. the bigger piece is extremely dangerous - imagine if they poke into the driver's eyes...
so, manufacturers improved it by creating tempered glass. tempered glass will break into thousand of small pieces to reduce injury in case of broken.
and then, as technology improved there is now so called laminated glass, also known as double layer as it is self explanatory. becoz of its design, it will hold to its shape even when beaten to shatters.
the downside is, such windscreen has its own weak zone and this kind of windscreen can crack easier than laminated glass but it is still safe to drive around. the crack can grow as we drive on uneven surface (for example) and that is a normal characteristic of it.