Full Idea
Popper's principles are roughly that one theory is superior to another if it has greater empirical content, if it can account for the successes of the first theory, and if it has not been falsified (unlike the first theory).
Gist of Idea
Good theories have empirical content, explain a lot, and are not falsified
Source
report of Karl Popper (The Logic of Scientific Discovery [1934]) by W.H. Newton-Smith - The Rationality of Science I.6
Book Reference
Newton-Smith,W.H.: 'The Rationality of Science' [RKP 1981], p.15