That's Latin for "Come, Holy Spirit." Tomorrow is Pentecost, so remember to wear red. It's the one Sunday of the year that we focus explicitly on the Holy Spirit. One medieval Latin chant that is appropriate for the Sunday still appears in the ELW, but we won't be singing it this week. Instead, it will be the meditative prelude. You might want to open to ELW 577 to follow along to the text there.
Here is one translation of the first verse, for our own prayers to be guided by the Holy Spirit:
Come, Holy Spirit, Creator blest,
And in our souls take up thy rest;
Come with Thy grace and heavenly aid
To fill the hearts which Thou hast made.