A life support device was any implement that helped a being stay alive, whether its support be permanent or dependent on the situation, such as surviving in settings that were polluted or otherwise not adapted to their natural life requirements.
Life support systems made the interior of a starship capable of supporting life in the otherwise deadly environmental conditions of space. One important function of life support was to renew the breathable air inside the ship. Other parameters included the ambient temperature.
Some crafts, like the Imperial TIE/ln space superiority starfighter, lacked such systems, which forced their users to wear life support chest pieces with breather tubes connected to the helmet to provide necessary gases. Similarly, Woan Barso wore a vac-suit at all times because he did not trust the life support of his dilapidated container tug.
Darth Vader's mask incorporated life support technology because, due to his injuries, he could not breathe on his own.
Individual life support implements took a variety of forms according to the degree of dependence of the beings using them. For example, Kel Dors like the Jedi Master Plo Koon only needed to wear a specialized breath mask and protective goggles so they could function in conditions different from those on their home planet of Dorin. At the other end of the spectrum, Anakin Skywalker was so gravely injured after his duel on Mustafar that he needed an armor offering total and permanent life support, without which he would die. Similarly, during the last days of their lives, the monarchs of Mon Cala Lee-Char and Ech-Char were maintained on life support.