The word sadhna comes from the word sadhya, which means to achieve; or to aim or focus. Any intense practice that is done with 100% focus is called sadhna.
Sadhna is a long path. To do sadhna requires great patience. Sadhna can be done of any subject, be it music, yoga, archery or any other practical art. The goal of sadhna is to become one with the subject; to reach a point where there is no distinction between the subject and the practitioner. For this, you have to focus all your energy on the subject that is to be mastered or achieved.
To become one with the subject, one must go to the beej or seed of the subject, from where it emerges.The beej of the subject is the purest state of the subject, it's root. For example, in music, the root of any instrument is pure sound or in the language of Vedanta - Aum. From there, everything that is music emerges. This beej of music – pure sound, like the beej of any other subject can be called by many names – Truth, Ultimate Reality, Existence, etc. Thus, if one goes to the root of a subject through sadhna, one experiences ultimate knowledge or Truth. So we can also say that the highest goal of sadhna is to experience Truth. The subject is the medium and gaining mastery over it a practical outcome. The sadhna or intense practice of a subject takes the sadhak or practitioner from the material level to higher spiritual levels, where he can ultimately go to the subject's root and experience Truth.
Truth is not something that can be taught. It is something that is be experienced or self-realized. Unfortunately, in today's society, the education practice is such that we are given ready-made information that we are to accept rather than discovering them through our own understanding. It is a system that creates more believers than seekers.
Truth needs no belief, Truth is being. It is eternal – shashwata. Even if Truth itself comes to you and tells you to believe, you must not believe. Because that belief will make that Truth a lie. Truth itself is not a lie, but your belief of it without experience is wrong. Truth never creates beliefs. Truth is Truth. When you have any experience, your belief turns into knowledge and that is Truth.
What are the obstacles that come in the way of experiencing Truth?
The biggest obstacle is your mind. Everything – your beliefs, ideas, concepts, thoughts and information are all the clouds that cover the Truth from you. Until you disconnect from these things, you cannot experience Truth, it can only be experienced in a mindless condition. It is through sadhna that one can reach a mindless condition.
As mentioned before, one can do sadhna of many different topics. But the ultimate goal is one and the same: the experience of Truth, which occurs in a mindless condition. The three most direct paths to reach a mindless condition are yoga, music and tantra. It is important to understand that there is not one single path or sadhna that everyone can do. Each person is unique and so their path is unique, but there are similar experiences that sadhaks share as they move towards a common goal.