Krishna Amavasya is the name for the 30th Tithi (New Moon lunar day). It's the 15th Tithi of Krishnapaksha. It's nature is Pitri Prada, which can be interpreted as "related to ancestors". It's good for following works: religious works; Yagya, donations, Agnihotra, Aganyadhan; ancestral works; meditation, activation of Mantras. All other auspicious works not related to ancestors will be destroyed if done during Amavasya.
Pitris is Devatas which rules Amavasya (referring to Muhurta Chintamani, Brihat Samhita and Poorva Kalamrita). Pitris are spirits of the departed ancestors and there are lots of Pitri forms and classes (devāḥ pitaraḥ, manuṣyāḥ pitaraḥ, etc). The main idea for Amavasya is that Pitris should be honored during this lunar day.
Although being Poorna (that can be translated as "full", "complete" or "perfect") and ruled by Guru (Jupiter), 15th Tithi of Krishnapaksha is suitable only for works that related to Pitris. Other works that related to Poorna Tithis (5th, 10th or 15th) are prohibited, as Chandra (Moon) will be combusted (the state that's called in sanskrit as Ashtami or Mushti), Tithi Gandanta, Maas Gandanta, Paksha Gandanta etc., will take time and place. Also it's a Parva Tithi which should be avoided during all auspicious elections. Being Galagraha Tithi it should be left in all Manglik elections.
During the vedic month Shravana Krishna Amavasya becomes Daiva Savarni - Manvadi Tithi, which is considered to be extremely auspicious for any Punya Karma (Japa, Tapa, Dana, Havan, Homa etc.). But being Manvadi Tithi Krishna Amavasya should be avoided in starting of education, Upanayana, marriage, ending of fasts, construction of house and entering thereof; travelling; plantation works.