One of my least favourite tools and takes a bit of practice, do read the docs though https://docs.gimp.org/en/gimp-tool-align.html
For your example: Looks like you want to center the red text but I will use a general example. Make sure the text layer is minimised in size - put the text in dynamic mode in the text tool options.
With the align tool active
1. For the base layer, click in the canvas. That gives squares in the corners (that is your "Relative to First Item")
2. Select the text layer and shift-click to select that. Again squares in the corners.
3. Click on the horizontal align icon in the align tool options
and as a further example,
4. You can repeat with a different align option.
For your example: Looks like you want to center the red text but I will use a general example. Make sure the text layer is minimised in size - put the text in dynamic mode in the text tool options.
With the align tool active
1. For the base layer, click in the canvas. That gives squares in the corners (that is your "Relative to First Item")
2. Select the text layer and shift-click to select that. Again squares in the corners.
3. Click on the horizontal align icon in the align tool options
and as a further example,
4. You can repeat with a different align option.