From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
Rejuvenation Tanks (メディカル マシン, Medikaru Mashin; Literally meaning "Medical Machine") are large machines filled with some form of liquid that can completely heal a person from near death.
When a person was badly injured, they were put into a Rejuvenation Tank and hooked up with a breathing apparatus and other various wires. After they healed, they left the tank fully rejuvenated. Because of the Zenkai healing abilities of the Saiyans, these tanks massively increased a Saiyans' power after use. It was assumed that this technology was developed by Freeza's Force, led by King Cold and his two sons, Cooler and Freeza.
 Dragon Ball Z
During the Namek Saga in Dragon Ball Z, we first see Vegeta use the tank upon his return to Planet Freeza No. 79 after being badly defeated on Earth by Son Goku. After he is fully healed, he heads for the planet Namek.
Vegeta later used the Rejuvenation Tank once more while on Planet Namek after he was badly injured in a fight against Zarbon. Zarbon took Vegeta back to Freeza's Spaceship and put him into the Rejuvenation Tank so that he could be healed and later interrogated. Vegeta broke out of the tank, however, destroying it in the process and stealing all of the Dragon Balls from Freeza.
When Goku was returned to his own body after Ginyu attempted to steal it, he was placed in an older Rejuvenation Tank because the newer one was destroyed by Vegeta. After he healed, Goku made his way toward the battlefield where he confronted Freeza.
Shortly after being rescued from death by his father, Cold, Freeza was seen using a rejuvenation tank prior to his receiving cybernetic parts to compensate for the loss of his limbs during his fight with Goku.
Rejuvenation Tanks appeared in Dragon Ball Z: Bardock: The Father of Goku. After being severely injured on Planet Kanassa, Bardock was taken back to Planet Vegeta and put into recovery. Later on, he healed and made his way to Planet Meat.
- Vegeta states once that the liquid in the tank is synthetic Saiyan DNA.
- Vegeta (twice), Goku, Bardock, and Freeza use Rejuvenation Tanks in the series.