This is a very common fold in origami where the paper is folded inside itself.
1) I’m starting out with a square folded in half diagonally
2) I’m going to make an inside reverse fold on the right side so I start by making a valley fold.
3) Crease the fold well and then unfold it.
4) Flip the paper over and fold the same crease from step 3 in the opposite direction along the same line you already folded.
5) Open the paper up somewhat and fold the section you just creased inside. Since you folded that section both ways it should easily tuck inside.
6) Push everything flat and you have an Inside Reverse Fold.