It is a special church service, different from a christening, can be used as a way to say thank you to God for your child.
It’s possible to have this service instead of a christening if you decide that now isn’t the right time to have your child baptized. Or, you could even have the Thanksgiving service as well as a christening.
You might also choose this service when your baby is just born, or if you have adopted a child into your family and you simply want to celebrate this with special words said by a vicar in church.