Complex sentence: Basic beliefs are beliefs that give justificatory support to other beliefs, and more derivative beliefs are based on those more derivative beliefs; but basic beliefs are said to be self-justifying, that is, they are justified but not justified by other beliefs.
Split into simpler sentences: Basic beliefs are beliefs that give justificatory support to other beliefs, and more derivative beliefs are based on those more basic beliefs. The basic beliefs are said to be self-justifying, that is, they are justified but not justified by other beliefs.