Complex sentence: SPDY does not replace HTTP, it modifies the way HTTP requests and responses are sent "over the wire"; this means that all the existing server-side applications can be used without modification if a SPDY-compatible translation layer is put in place.
Split into simpler sentences: SPDY does not replace HTTP; it modifies the way HTTP requests and responses are sent "over the wire". This means that all the existing server-side applications can be used without modification if a SPDY-compatible translation layer is put in place.