Palms are among the most expressive parts of the body and is used for most kind of greetings (from waving your palm to say “hi” to handshakes to Indian namaste). Indian dance forms focus a lot on “hasta mudras” – where the dancer users his/her palms to convey deep philosophical concepts.
Keeping palms together signals welcome and union in a symbolic way.
In India, touching any part of others body (especially with people of opposite sex) always had a taboo (except when it is used to touch the feet). So handshakes are out. This is probably to do with the sanitary aspects in a tropical, agrarian society where contagious diseases are always around.
Article Source: https://www.quora.com/What-is-the-origin-and-significance-of-joined-hands-used-in-Hinduism-as-a-form-of-greeting-prayer
It is also called the Namaste gesture.
Namaste is derived from Sanskrit and is a combination of two words, “Namaḥ” and “te” (a shortened variant of “tubhyam”). Namaḥ means ‘bow’, ‘obeisance’, ‘reverential salutation’ or ‘adoration’ and te means ‘to you’ (dative case of ‘you’). Therefore, Namaste literally means “bow to you” translated as “I bow to you”.
Namaste happens to be both a formal and an informal form of greeting in India and you can say this to anyone irrespective of age. Normally when you say Namaskar to anybody, you press both your palms together with all the fingers pointing upwards in front of your chest. At the same time, you also bow your head slightly, looking at the person you are saying Namaste to. Even if you simply perform the Namaste gesture with your hands without actually saying the word, it will mean the same thing.
Though saying Namaste to others in daily lives is a part of the Indian protocol, yet many believe it also has religious / spiritual connotation. According to this school of thought, when you greet Namaste, you actually seek to recognize a common divinity within the other person. Interestingly, Namaste can be said in different ways, depending crucially on the person you are saying it to. For instance, when you greet your friend or peer, the traditional style Namaste will suffice.
On the other hand, when it’s a person greeting Namaste to another person of a higher status, this gesture will get intensified. To indicate genuine and deep respect for the other person, you place the hands in front of the forehead. Whereas, you have to pay reverence to God or a holy person then, this feeling can be transmitted through the Namaste gesture by holding / placing the pressed hands above the head. Some natives prostrate on the ground in this posture to show their deep respect and love to God.
The symbolism of the two palms touching each other is of great significance. It is the joining together of two extremities – the feet of the Divine, with the head of the devotee. Yet another theory associates the Namaste greeting with a particular mudra or posture in yoga. However, Namaste being a polite gesture of love and respect can be said to anybody. But traditionally, it’s a Hindu gesture and people of this community greet each other this way only.