Sensus plenior is used by different people in different ways. The most common meaning used in debates among Catholics and later among Evanglicals is the one offered by Raymond Brown:
The sensus plenior is that that additional, deeper meaning, intended by God but not clearly intended by the human author, which is seen to exist in the words of a biblical text (or group of texts, or even a whole book) when they are studied in the light of further revelation or development in the understanding of revelation. Raymond E. Brown, The Sensus Plenior of Sacred Scripture (Baltimore: St. Mary's University, 1955), p 92
Sensus plenior resembles allegory, but those who use it say it is different because there is a strict set of rules.
For more nformation see: What is Sensus Plenior and how does it impact the field of hermeneutics?