Just as you and I can see through windows, birds can as well. Unfortunately, the difference is that the birds do not realize that there is a pane of glass. This sadly leads to many injured birds and fatalities, as birds fly at top speed and hit the glass head-on.
We can help our feathered friends by placing silhouette cutouts of raptors on the windows, or draping a fine netting over the glass. Both of these serve to break-up the view, and deter the birds from flying that way.