What Does "In Bending" Mean?

This looks as though it pertains to gear/rack teeth. If you push the whole tooth axially, it will try to slide along the root, which is a shearing action. (The force is "in shear.") If you push the tooth at the top, it will try to lean over, or bend. (The force is "in bending," and it involves leverage rather than sliding alone.) This is a rather basic verbal summary, but I hope it helps.

Bending materials means that you press them into another shape. Every material will resist this manipulation; every material will also react differently.

So if you bent a material, say to a 90° angle and you then remove it out of the bending equipment it will stretch back to e.g. an opening angle of 92°. If this is within tolerance it is OK, but if you need the 90° angle as an end result you will have to make an "in bend" (angle smaller than 90°) to obtain 90° angle as end result.

How much this "in bend" effect has to be is depending on the material (stainless steel, Aluminium, alloy) and the thickness. The best result you get when you test the combination of material and bending equipment, pressure during bending process, time of bending

does "in bending"mean "when subjected to bending"

The question was just about the terminology, in this context the answer is yes 'in bending' means when subjected to bending.

Finally, a sane and simple answer...