Unit step

A unit step function is defined as:

$$u(t)=\{\begin{array}{ll}1& t\ge 0\\ 0& t<0\end{array}$$

If we want any signal to start at $t=0$, we can just multiply it by the unit step function.

We can also describe rectangular pulses using delayed unit step functions. For example:

$$x(t)=u(t-2)-u(t-4)$$

The left side shows the resulting rectangular pulse, while the right side shows the two step functions separately to illustrate the operation at hand.