Tmux can be a pretty complex piece of software, but iTerm can basically wrap it all up into a nice package. You don't need to know anything at all about tmux to use iTerm's tmux integration.
If you're SSHed into a server that has tmux installed, try running `tmux -CC` on the server. It'll pop up a new window that looks and feels just like it's running natively (complete with tab support), except it's all tunneled over SSH.
And if you disconnect, you can just reconnect later and your windows will all come back in the same state as when you left them.
I use this one-liner to SSH into a server and reconnect (or start) a tmux session:
ssh -XY -F user@hostname -Ct \
'sh -l -c "exec tmux -CC -u new-session -AD -s remote"'
iTerm2's tmux integration makes SSH access to remote machines feel almost as native as using your machine locally. It's really a killer feature, and it made iTerm2 worth a donation for me.