A Lorentz scalar is unchanged under a Lorentz transformation. Contracting every Lorentz index in a tensor expression produces a scalar when the constituent fields transform covariantly.
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In the context of special relativity, a Lorentz scalar is a quantity that remains invariant under Lorentz transformations, which relate the physical quantities measured in different inertial reference frames. To elaborate, a Lorentz transformation is a mathematical operation that accounts for the effects of relative motion at speeds close to the speed of light, specifically how time and space coordinates change for observers in different inertial frames.