Hourglassing is an instability that happens in reduced integrated elements (Typically, quad4 and hexa8 elements). The reduced integration has two benefits 1) faster and 2) can give better bending results (less stiff), but it can also produce a shearing mode that has no strain energy associated with it and as a result the elements can form an hourglass shape with no strain energy - looks something like this
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See it looks like an hourglass.. This is easily removed by either selective reduced integration where the shear mode is fully integrated or by the addition of hourglass stabilization (see one of the many papers by Belytschko)
