When you log into a remote machine you may want to use SSH or SCP from there too. You could go through the process of copying your public and private keys to that machine and setting up an agent, but SSH can simplify this by forwarding your agents, so that when you SSH to a remote machine an agent springs into life with you keys set up ready for use. To have this happen, on your local machine, you have to create or update the file:-
$HOME/.ssh/config
and, for each remote machine you want to forward your agent to you have to add:- two lines:-
Host remote-machine ForwardAgent yes
e.g.:-
Host ppslgen.physics.ox.ac.uk ForwardAgent yes