Shear locking is an artefact in structural calculations by finite element methods when the structure looks like a slender beam or shell, the main deformation is bending and there is not enough elements accross the thickness or elements are significantly distorted (large aspect ratios). If this occurs then the stiffness of the structure is significantly (orders of magnitude) overpredicted (deformation is underpredicted) and the shear stresses are predicted much higher than the tensile stress (usually for bending is the other way around). Shear locking occurs than elements are not smart enough to use correct shape for shear stress inside the element (must me parabolic but many elements only consider stress to be linear inside the element or even constant) and the difference between the real and approximated by finite element shear stress profile is high. Then the finite element analysis might predict the structure as supported by large shear stresses (artefacts) rather than the tensile stresses.
Only 8node bricks, 4 node tets and 4 node quadrilaterals are susceptible to shear locking. Thus, if you use elements with midside nodes you should not worry about the shear locking. Also the 8 node bricks with "compatibility of deformation not enforced" (type 8 elements) are specially designed to not produce shear locking. If my memory serves me well they are the default settings for brick elements. You should not worry about shear locking if use those elements.
On the other hand 8 node bricks with "compatibility of deformation enforced" (type 5 elements) and 4-node tets are supectible to shear locking and should not be used for predictions of bending of slender beams and thin shells.
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