Skewing (also called shearing) is technically rotating the X and Y axes of an object differently. The result looks sort of like a 3D perspective effect. InDesign only skews horizontally, but it also fakes a vertical shear (or one on some other angle) by combining skewing and rotating.
To horizontally skew an object in the Transform or Control palette, enter an angle in the Shear Angle field, or choose one of the preset values in the popup menu. Both the object and its contents are slanted on the horizontal axis (unless you’ve turned off the Transform Content option in the palette’s flyout menu). Note that a positive shear value angles the object clockwise (the top shifts to the right), unlike the rotation tools where a positive angle is counterclockwise.