IPv6 allows, and often requires, multiple addresses on the same interface. The is more difficult in IPv4, although not always impossible.
What it means is that a host configured with multiple addresses on a single interface will receive traffic destined for any of those addresses. From the perspective of a layer-4 protocol, e.g. TCP, the host can have separate connections using the same TCP address (port) on the same interface because the IP addresses are different.
There are many reasons that this may be configured. If the networks are different, then someone may want the host to serve different networks, e.g. DHCP. If the networks are the same, then it may be due to the obfuscation of IP addressing by Privacy Extensions and Random Addressing that many OSes now employ.
With Cisco, to use multiple IPv4 networks on the same router interface, you must use secondary addressing, and this has some routing protocol limitations. With IPv6, you can simply assign different networks to the same interface, and the routing protocols will work with them all. This doesn't replace the subinterface concept that is often used for VLANs because the different networks will all be on the same layer-2 broadcast domain.