Prefixes are used with IP addresses. There are two parts to IP addresses. There’s the network portion, and there’s the host portion.
We use a subnet mask to show which part of the address is which.
An IPv4 address can use a Dotted-Decimal-Notation mask or a CIDR mask. In IPv6, we only use CIDR masks, because the addresses are so long.
In either case, the mask shows how much of the address is the network, leaving the rest to be the host.
This network portion is also called the prefix. This is the correct terminology when we’re talking about IPv6 addresses. It’s also a heavily used term when we’re talking about BGP, which can also carry non-IP address families.
In short, a prefix is the network portion of the address.