Every raag has a centre of gravity, and that centre is the vadi. The vadi is the note the artist returns to most often, lingers on longest, and approaches with the most ornament. It is the emotional anchor of the raag.
The samvadi is the vadi's partner — the second-most-emphasized note. By convention, the samvadi sits a perfect fourth (4 scale degrees) or perfect fifth (5 scale degrees) away from the vadi. This interval creates a satisfying melodic relationship: the vadi states the raag's mood, and the samvadi answers it.
In Yaman, the vadi is Ga (the third) and the samvadi is Ni (the seventh) — a fourth apart (since Ni is the seventh of the octave, four scale degrees up from Ga across the octave). In Bhairav, the vadi is Dha (sixth) and samvadi is Re (second). In Marwa, vadi is Re, samvadi is Dha. The choice of vadi-samvadi shapes how the raag feels — vadi in the lower octave gives one character; vadi in the upper octave another.
Beginning students of a raag are taught the vadi-samvadi alongside the thaat and aroha-avaroha. From the first phrases of alap, the artist's emphasis tells the listener which note is the vadi — and from that, everything else can be inferred.
The vadi-samvadi distinction is sometimes also linked to time of day: raags with vadi in the upper tetrachord (uttarang) are typically evening/night; raags with vadi in the lower tetrachord (purvang) are typically morning.