Dmenu is a dynamic menu for X often used on minimal window managers setups. It is part of the suckless tools suite of free and open source utilities created by the suckless.org project, which include, among the others, the dwm window manager and the st terminal emulator.
window
Introduction to terminal multiplexer Tmux
Tmux is a terminal multiplexer: it let us run and manage multiple terminal sessions from a single screen. This is specially useful when connecting to remote machines using ssh, since, among the other things, it allows us to keep processes started from those terminals running in the background when we disconnect from the session (or logout and close the remote secure shell altogether), letting us re-attach to it at a later time.