If your ancestors were Catholic, their christenings and marriages were witnessed by sponsors (also called Godparents in the case of christenings or simply witnesses at marriages). They were often relatives of the parties in question and genealogists collect information about relatives. The problem is that Catholic church register indexers have often chosen not to capture information about such witnesses. This means that if you want to search for who your ancestor was the Godparent of, or whose marriage they witnessed, your only option is to read the whole register from one end to the other - something that not many of us are likely to have time to do!
In the case of my own Catholic ancestors who were Irish immigrants who settled in Liverpool. I have sometimes found my ancestors acting as sponsors by chance, and it has often added useful evidence about the wider kinship networks of the family. It has left me wondering how many others I have not found which would help me build more evidence to support my family tree. That's where the Catholic Witness Index (CWI) comes in. It allows us to fill the gap by providing a website on which Catholic witnesses can be indexed.
But we need volunteers to help as there are lots of records out there. HistoryOwl is hosting this project on its website. Why not learn about how you can get involved? You might even bag some new information about your own family in the process.