Modulation, in music, means that the music changes key. A piece of music might, for example, be "in the key of C major" (meaning that it uses the notes of a C major scale, and the C sounds like the "home key" or "tonic" as it is called in music theory). Then it could modulate to G major so that the G now feels like the home key and the notes of a G major scale are used (the Fs will be F sharps).