tmux allows you to have multiple panes (the equivalent of windows in Vim) and windows (the equivalent of tabs). To access tmux functionality, you first need to hit a prefix key, followed by a command. The default prefix key is Ctrl + b.
You can rebind the default prefix key to something else by creating or editing your ~/.tmux.conf file. For example, if you wanted to use Ctrl + \ as a prefix instead of Ctrl + b, you would add the following:
# Use Ctrl-\ as a prefix.
set -g prefix 'C-\'
bind-key 'C-\' send-prefix
Restart tmux (or execute Ctrl + b followed by :source-file ~/.tmux.conf) to apply the configuration.
To split the screen vertically, use Ctrl + b followed by %:
For some reason, I could ...