"Scripture interprets Scripture" is the principle of Regula Fidei from the perspective of Sola Scriptura. However, it's also a mind-set that is used through many hermeneutic appraoches.
Sola Scriptura/Regula Fidei
The idea behind Sola Scriptura is that the Bible is complete and sufficient for all knowledge of salvation and holiness. From this doctrine, the Bible should be used for judging faith and soundness of doctrine and practices; it is, therefore, the "Rule of Faith", or measure by which we judge something.
"We believe that the only rule and standard by which all dogmas and all doctors are to be weighed and judged, is nothing else but the prophetic and apostolic writings of the Old and New Testaments"
(Form. Concordiae, 1577)
Scripture interprets Scripture
From this idea that scripture alone was sufficient came the idea that we should interpret scripture using other scripture. Since the Bible alone was the sole measure for judging faith and practice, it should be used as a measure to understand other parts of scripture.
As a Hermeneutic Approach
As a hermeneutic approach, "Scripture interprets Scripture" is the idea that we should read a passage in the light of the entire Bible. It also states that we should interpret confusing passages based on clear passages.
"Scripture interprets Scripture" is, in itself, a hermeneutic approach. However, it's also a mind-set used by other hermeneutic approaches.
Other Hermeneutic Approaches
This idea that scripture can be used to gain insight into other passages in the Bible is found through many hermeneutic approaches.
Sensus Plenior is one example that uses "Scripture interprets Scripture" as a general guiding rule. Theological Analysis is a technique that shows that theology must harmonize throughout all the scriptures (so that one small passage doesn't create doctrine).
The idea that we can use scripture to interpret other scripture is a common theme throughout Hermeneutics. From the perspective of Sola Scriptura, it is, by itself, a hermeneutic principle.
Ultimately, it is simply the idea that we can use scripture to shed light on other passages within the Bible.