Whilst they are really good  uPVC windows are not 100% soundproof and it depends on the area you live in but they will allow you to keep most if not all unwanted outdoor noise out, in fact double glazed uPVC windows are able to cut down noise by as much as 70%.

The reason why UPVC windows help to reduce exterior sound is their construction.

The frame itself is a poor conductor of both heat and sound as it is rigid and has low thermal conductivity.  This means that sound struggles to pass through the material into your home.  

The glazing panels which are installed to provide double glazing feature two separate layers of glass with an air gap between the panes.  This gap is usually filled with regular air, but some panels use different gasses such as Argon or Krypton depending on the specific performance requirements.  The air gap acts as a sound barrier, creating an extra interface for sound to be transmitted and reducing the amount of noise that is carried through into your home.