In other words, for a typical layer, this will 'fade it out'. In the Parameters Panel, look for an option labeled Amount - this controls how much of the blended result of the layer is composited with the blend of the layers beneath it. Show or hide a layer, or fade the effect of a blur?
You can also create some text from your svg software preferred editor ( inkscape for example), create some text and import it into synfig. Use the Text Tool or right click on your Canvas and choose " → → Other → Text".