It means "Here I Am, Lord", and is the title of a popular hymn. The hymn goes like this:
1.
I, the Lord of sea and sky,
I have heard my people cry.
All who dwell in dark and sin,
My hand will save.

Chorus
Here am I, Lord. Is it I, Lord?
I have heard you calling in the night.
I will go, Lord, if you lead me.
I will hold your people in my heart.

2.
I, who made the stars of night,
I will make their darkness bright.
Who will bear my light to them?
Whom shall I send?

Chorus

3.
I, the Lord of snow and rain,
I have borne my people's pain.
I have wept for love of them.
They turn away.

Chorus

4.
I will break their hearts of stone,
Give them hearts for love alone.
I will speak my words to them.
Whom shall I send?

Chorus

5.
I, the Lord of wind and flame,
I will send the poor and lame.
I will set a feast for them.
My hand will save.

Chorus

6.
Finest bread I will provide,
'Til their hearts be satisfied.
I will give my life to them.
Whom shall I send?

Chorus
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However, I don't know what Cerys Matthews is singing tho; —just thought I'd translate the title, and then got side-tracked...