"ecs.dnsp.co" tells you whether your resolver disclosed part of your IP address to us.
EDNS Client Subnet (RFC 7871) exists so that content delivery networks can answer with a server near you rather than near your resolver. To do that, your resolver copies some of your address into the query it forwards. It is a genuine trade: better routing in exchange for telling every authoritative server you touch roughly where you are.
If this test reports no client subnet, your resolver is keeping your address to itself and only its own address is visible to us. If it reports a subnet, that prefix is what every authoritative nameserver you query can see — not just this one.
prompt> dig +short ecs.dnsp.co TXT "No EDNS Client Subnet in this query. Your resolver did NOT disclose any part of your address to this server. This is the private behaviour."