In the Naruto series, the titular protagonist trains with the great sage elder toad Fukasaku on Mount Myōboku in order to learn senjutsu techniques and defeat the villainous Pain. Senjutsu is the combination of natural energy with the user's personal chakra to use powerful abilities called Sage Art.
To draw the natural energy within, the user must first enter Sage Mode. For the untrained this requires the user to become very still, adopting a meditation pose. This pose seen in Naruto actually uses a Buddhist mudra from real life, called "Mida no Jōin."
Mudras are ritual or meditative hand gestures in certain religions such as Buddhism, and they each have a different pose, meaning and story behind them. Mida no Jōin used in Sage Mode training is famously seen by The Great Buddha of Kamakura meditating with the exact same mudra as in Naruto's Sage Mode training. This mudra is also represented by the Buddha Amida, or Amitābha, in places outside of Japan such as Korea and Mongolia.
Buddha Amida, also known as The Buddha of Infinite Light, is a highly important figure in Buddhism around the world. The Mida no Jōin pose represents the victory of enlightenment over this world of illusions. The purpose of meditation in most religions or arts is mindfulness and to be connected with the natural world. This is what Naruto's Sage Mode and senjutsu is all about. Sage Mode utilizes the natural world's energy and infuses it with the user's chakra, allowing a person to become one with nature.
In Naruto, being in Sage Mode increases one's physical abilities such as strength, speed and stamina. The user is also able to sense surrounding chakra -- including attacks -- without having to visibly see them. This gives the user superhuman reflexes to be able to dodge attacks coming from blind spots. Sage Mode also allows for transformation, which gives a user distinctly powerful physical abilities and new techniques.
Only a few of the strongest characters in Naruto have ever mastered Sage Mode, including Jiraiya, Orochimaru, Kabuto, Mitsuki and Naruto himself. Only the most powerful have the ability to learn senjutsu; if a user does not have the ability to handle the fusion and balance of natural energy with their own chakra, they could permanently transform into an animal such as a frog or a snake.