“The Roman lawyers of the sixth century were, however, just as mindful of the need to educate their successors as their predecessors had been and indeed, members of the legal profession are just as concerned today. Justinian therefore asked Tribonian and two of his professional colleagues, Theophilus from Constantinople and Dorotheus from Beirut, to prepare an elementary textbook for students from which they could learn the principles of the law and through which they would be guided to the Digest itself. Taking the Institutes of Gaius as their model, they produced the Institutes of Justinian which was also published in December 533 and given the force of law – which soon proved inconvenient, for again the cross-referencing was not very efficient and at times rules are stated different in the Digest and Institutes...”
So, the Institutes were the textbook which was meant to guide a young Roman toward the Digest, which was the law as written. As a side note: Justinian dedicated the Digest to God, and considered it his task to deliver stability as guided by God. He also forbid all commentaries on the Digest, thus making the law enshrined and trying to make it impossible to default to earlier versions of laws superseded by the Digest. That's... a lot of civil power, when you think about it.