The -J and ProxyJump options are a way to connect through a specified host, to a host beyond.
The manual entry from the official manual pages (https://man.openbsd.org/ssh) for -J [user@]host[:port] is as follows:
Connect to the target host by first making a ssh connection to the jump host described by destination and then establishing a TCP forwarding to the ultimate destination from there. Multiple jump hops may be specified separated by comma characters. This is a shortcut to specify a ProxyJump configuration directive.
The manual entry from https://man.openbsd.org/ssh_config for ProxyJump is as follows:
Specifies one or more jump proxies as either [user@]host[:port] or an ssh URI. Multiple proxies may be separated by comma ...
