In Grow when you select your data series, Grow will automatically select the series title from the first row of the column.
You can rename this title to whatever you want.
Sometimes, your series labels are not present in the first row, but are present in the data.
Add Value Titles allows you to dynamically select the series titles from the data.
Your titles need to be sequential and in the same order as your data series.
Just click Display > then pick 'Add Value Titles' and select the cell or cells that you want.
The data series titles are displayed on the chart legend, as well as in the mouseover tool tip.