Karmawibhanga Museum sits within the Borobudur temple complex and helps explain details that are easy to miss when the monument itself overwhelms everything. Its focus includes the hidden Karmawibhanga reliefs, restoration work, archaeological fragments, and the long modern effort to preserve the temple.
It is a supporting visit, not a substitute for Borobudur. Go after walking the temple if you want to understand what was reconstructed, what remains hidden, and how much labour sits behind the clean stone terraces visitors see today.