Healing in the Classic Games is only manually by using Medipacks, while in the Reboot Games Healing is generally automatic: when Lara Croft is injured and takes no further damage she will heal herself within the next couple of seconds.
In Rise of the Tomb Raider and Shadow of the Tomb Raider it nonetheless is necessary in some situations to manually heal Lara, e.g. during combat when she can't take cover for long enough. Note that, depending on the Difficulty settings, she might not heal automatically at all.
Healing uses up Resources from Lara's inventory: One item of Herbs (Rise of the Tomb Raider) or Medicine (Shadow of the Tomb Raider) and one item of Cloth is used up during the process. Healing takes a small amount of time, around one or two seconds, which may be enough for an enemy to kill Lara, so it is advisable to heal her sooner that later.