This can mean several things (like simply making sure the rhythmic meter of the music matches the rhythmic meter of the music, or matching happy lyrics with up-beat, major key music and sad lyrics with slower, minor key music), but our focus today is on tracking music and lyrics together. Put another way, the music matches the lyrical content (or the lyrical content matching the music). This is also sometimes referred to as "word painting".
For example, when a Sly Stone sings "Gonna take you HIGH-ER" and the pitch rolls up on the last word....or when Garth Brooks sings "I've got friends in LOW places" and drops the pitch on the word "low".....or when Bruno Mars sings "Stop - wait a minute" and the entire band stops playing.....or, for a more subtle example of "stop", when Diana Ross songs "Stop - in the name of love" and the melody stops dead after the word.....or when a vocalist sings "Going down - down, down, down, down, down" in the old Don Nix blues standard "Going Down" (covered by Freddie King, Jeff Beck, Beth Hart, Joe Satriani, Gary Clark Jr, and others,) each "down" drops further in pitch, taking the ear lower and lower.