Taras are used to assess the favorability of a particular Nakshatra for initiating an activity. They are derived by counting from the native's Janma Nakshatra to the Nakshatra being transited by the Moon (or sometimes other planets) on the day of the activity. The count is then divided by 9, and the remainder determines the Tara.
Taras are grouped into auspicious and inauspicious categories:
Auspicious Taras (for starting new projects): Janma (mixed), Sampat (2nd), Kshema (4th), Pratyari (5th - usually avoided for major events), Sadhaka (6th), Mitra (8th), Parama Mitra (9th).
Inauspicious Taras (to be avoided for important events): Vipata (3rd), Pratyari (5th - especially for important matters), Vadha (7th), Naidhana (7th), Pratyari (5th), Adhi Mitrasya (8th), Mitrasya (9th), and Vainaasika (22nd).
These are positional relationships from your natal nakshatra (Moon’s constellation). The count is forward in the nakshatra list (Ashwini → Revati).