# Use of step function to show that total charge is Lorentz invariant if the four-gradient of current density is zero

I am reading the chapter on Special Relativity in Steven Weinberg's 'Gravitation and Cosmology'. It is stated in the book that total charge can be written as $Q=\int d^4xJ^{\alpha}(x)\partial_{\alpha}\theta(\eta_{\beta}x^{\beta})$ and where $\theta$ is the step function, and $n_1=n_2=n_3=0; n_0=1$. I cannot make sense of the use of step function and the further steps in proving that total charge is an invariant scalar under Lorentz transformations. Can somebody please help me with this?