What are swaras?
Hindustani classical music has seven basic notes called swaras. These are Shadja, Rishabha, Gandhara, Madhyama, Panchama, Dhaivata and Nishada. We commonly come across them in their shortened forms as Sa, Re, Ga, Ma, Pa, Dha, and Ni. Sa is the root/base note and the reference point based on which an octave is defined. Together, they are known as the sargam.
The Shadja or Sa is the root/base note with respect to which remaining swaras of the raga (tune) are defined. Any raga in Hindustani classical music is a journey of notes that start from this home called Sa. The path that we decide to take for this journey from home is never the same for two different ragas. It is distinct in many ways. A musician performing Indian classical music can improvise in any way, as long as they stick to the predefined pathway of a raga .
The Shadja (Sa) or base note is so important to Hindustani classical music that one of the India’s greatest vocalist, Bharat Ratna Pandit Bhimsen Joshi once exclaimed …
“Taal gaya toh baal gaya, lekin sur gaya toh sir gaya”
which translates to “Going off beat is like losing your hair, but going off pitch is like losing your head”.
Why practice swaras?
The following video by Ms. Anuja Kamat  explains in depth why practicing swaras to perfection is so important for a singer.
Sources referred to in this article  - This quora article on svaras is informative.  - Anuja Kamat is a popular youtuber who explains concepts of Indian classical music
Also published on Medium.