Network interface name has with at sign - what is it?

eth0@if10 means:

• your network interface is named still simply eth0 and network tools and apps can only refer to this name, without the @ appendix. (As a sidenote, this is most probably a veth peer, but the name does not need to reflect this.)
• @if10 means: eth0 is connected to some (other) network interface with index 10 (decimal). Since there is also a link-netnsid 0 shown, this other network interface is in another network namespace (kind of virtual IP stack), presumably the root (a.k.a. host) network namespace.

If you use ip link show in your host, and not in your container, then one of the network interfaces listed there should have an @9 appendix; the interface name will probably start with veth.... This interface is the peer to the eth0@10 interface you asked about. Veth interfaces come in pairs connected to each other, like a virtual cable.

So, the @... is an appendix created by the ip tool, and it is not part of Linux' network interface names. The number after the @ refers to another network interface with the index number that is shown after the @. The index numbers are printed before the network interface names, such as in 9: eth0@if10. The peer network interface can be in a different network namespace.

Unfortunately, finding the correct network namespace for the link-netnsid .. is rather involved, see how to find the network namespace of a veth peer ifindex.