What is signal-energy?
Pick up an apple and raise it one meter. You have just spent (approximately) one joule of energy. You had to spend energy because the action (raising the apple) goes against a force (Earth's gravity).

The same is true for electrons. An electrical signal is nothing but electrons moving in an electric field. Causing these electrons to move takes energy, exactly like moving a train or an apple.

In the formula you posted, $x(t)$ is a voltage causing electrons to move in a resistance of $1\,\Omega$, and $E_s$ is the energy required to make those electrons move around in the resistor, causing a current to develop.