It's a series of textbooks [1] based on lectures Feynman gave while teaching the introductory physics sequence at Caltech.

To my knowledge, no universities actually use the Feynman Lectures as a textbook for their introductory courses. However, some students who have already taken physics read the Feynman Lectures later and (self-)report a much clearer understanding.

[1] https://en.wikipedia.org/wiki/The_Feynman_Lectures_on_Physic...