Connect via a server relay with target. Useful if target is behind a firewall, but reachable from relay.

ssh -J user1@relay user2@target

This can be combined with other options. This way a port forwarding can be established over which, e.g., sshfs can be used.

ssh -L 9999:localhost:22 -J user1@relay user2@target
sshfs user2@localhost:/path /mountpoint -C -p 9999