A subdomain is a prefix attached to the front of a domain name. If localnode.app is the domain, then myproject.localnode.app is a subdomain. They are fully independent from the web server's perspective - you can point them anywhere, to any IP, to any service.
Why they matter
Buying a new domain for every side project or home server gets expensive fast. Subdomains give you a clean, memorable address without the cost. Instead of bookmarking 192.168.1.42:8096, you get something like jellyfin.localnode.app. Much nicer to type, and it actually works from outside your house too.
The DNS side
Every subdomain is ultimately just a DNS record - usually an A record pointing to an IPv4 address, a AAAA record for IPv6, or a CNAME pointing at another hostname. Once the record is in place and propagated (usually a minute or two on Cloudflare), the subdomain is live worldwide.
Getting started on localnode.app
Sign up, pick a name, and your subdomain is live within seconds. You get up to five subdomains on the free tier - enough to give each of your home services its own address without any of the usual hassle.