In brief
text-shadow adds shadows to text using offsets, blur radius and colour. It differs from box-shadow, which shadows the box.
Useful to know
text-shadow adds shadows to text using offsets, blur radius and colour. It differs from box-shadow, which shadows the box. [1]
For a label over a photograph, a shadow can help visually but does not guarantee readability over every image. Check the lightest and darkest areas and provide a suitable backing when the text is essential.
Sources
For further detail and to check the information: