In Ghost of Tsushima, hot springs are an opportunity for Jin to reflect on events, characters, and trivial things as he enjoys his well-deserved bath. You can choose what to meditate upon at the beginning of the bath, but note that this has no influence on the story or the amount of max health unlocked.
There are a total of 18 sources to find, you can choose to invest one talent point in the Health Wind to guide you to them, or use our guide.
Finding several hot springs at the beginning of the game will help you greatly to progress, as death can comes very quickly. Their impact is particularly important for duels.
If you wish to search for the springs by yourself, you can clean Mongol camps, which will reveal the surroundings, or spot the steam columns and the characteristic red maple.
Bathing in the 18 springs unlocks a unique armor, the Fundoshi, in addition to health boosts. It will allow you to sprint without making any noise.