What it is
Finds subdomains through public Certificate Transparency logs, via crt.sh. Every certificate a CA issues is logged publicly, so any hostname anyone has ever requested a certificate for ends up on record. Any address on the domain works, so www.example.com and shop.example.com both list every subdomain of example.com.
Why it matters
Subdomains are where the forgotten things live: staging servers, admin panels, old campaign sites, a dashboard somebody stood up for a demo and never took down. They tend to be maintained to a lower standard than the main site. CT logs are historical, though, so expect a good share of what turns up no longer to resolve.