Colostrum is the first milk that comes out of the breasts of an individual after giving birth. Colostrum is high in protein and low in fat and sugar, containing a lot of antibodies which help in strengthening and developing the baby's immune system. It is thick and slightly yellow is color and a small quantity provides exceptionally high nutrition to the baby. This is the first stage of the milk, eventually (that is, within 2-3 days) this changes to the next stage, known as transitional breast milk which is a combination of colostrum and mature breast milk which then changes into the final stage which is mature breast milk.
Sources:
https://my.clevelandclinic.org/health/body/22434-colostrum
https://www.verywellfamily.com/colostrum-the-first-breast-milk-431990
https://www.healthline.com/nutrition/bovine-colostrum#nutrition