Examples of scalar products are
$$
\begin{aligned}
&p^{\mu} x_{\mu} \equiv p \cdot x=E t-\mathbf{p} \cdot \mathbf{x} \\
&p^{\mu} p_{\mu} \equiv p \cdot p \equiv p^{2}=E^{2}-\mathbf{p}^{2}
\end{aligned}
$$
These quantities are Lorentz invariants. For a free particle, we have $p^{2}=m^{2}$, see (3.10). We say that the particle is on its mass shell.