Netfilter is an open-source project that is used to perform packet filtering, aka the linux firewall.
This project is often mentioned iptables which is the userland application used to configure your firewall. In 2014, a new subsystem was added to the netfilter firewall, called nftable, which is now configured through the nftables userland application.
The core logic of netfilter is implemented within the kernel (/net/netfilter), and the userland tools communicate with it via netlink messages.