Purva Mimamsa opens with one of the most quoted lines in Indian philosophy: athato dharma jijnasa, now therefore the inquiry into dharma. Jaimini's Mimamsa Sutra sets the school's whole agenda in that sentence. Dharma, for Mimamsa, is a specific, knowable set of duties, above all the Vedic sacrifices, and the school works out exactly what those duties require and why the instructions that specify them are authoritative.
The authority question is the hard one. A Mimamsaka cannot say the Veda is reliable because a trustworthy person composed it, since the school holds the Veda has no human or divine author at all, apaurusheya, and no beginning in time. Instead Mimamsa built a theory of language and testimony, shabda, to explain how words carry binding authority on their own, focused above all on the injunctive form, vidhi, the grammatical shape of a command.
That focus produced apurva, the unseen potency a properly performed ritual generates and stores until it ripens into its promised result, sometimes long after the rite is finished, Mimamsa's way of explaining how an action can have consequences not immediately observable.
Two commentators, both active in the seventh century, shaped how later readers understood all this. Kumarila Bhatta and Prabhakara agree on the school's basic commitments but disagree on how a cognition certifies its own reliability, and their disagreement split later Mimamsa into two named sub-traditions. Later tradition reads Purva Mimamsa as the necessary first half of a two part inquiry, with Uttara Mimamsa, better known as Vedanta, taking up the Upanishads once the ritual Veda has been accounted for.