The set-methods will replace the current Matrix with new values, disregarding whatever the Matrix contained before. The pre and post method will apply a new transformation before or after whatever the current Matrix contains.
In this example, the rotation will be ignored since we are using the set method and the m will only contain a translation:
Matrix m = new Matrix();
m.setRotate(90);
m.setTranslate(100, 100);
In this example, the final matrix will be a translation followed by a rotation:
Matrix m = new Matrix();
m.setTranslate(100, 100);
m.postRotate(90);
In the final example, the final matrix will be a rotation followed by a translation:
Matrix m = new Matrix();
m.setTranslate(100, 100);
m.preRotate(90);
There is some more information in this (rather lengthy) post:
https://medium.com/a-problem-like-maria/understanding-android-matrix-transformations-25e028f56dc7
Hope it helps.