The weak derivative of a locally integrable $u$, denoted $u'$, is a function such that $\forall \varphi\in C^{\infty}_0(\Omega),\quad\int_{\Omega}u'(x)\varphi(x)\lambda(dx)=-\int_{\Omega}u(x)\varphi'(x)\lambda(dx),$ where $\Omega$ is the open on which we are working and $C^{\infty}_0(\Omega)$ is the vector space of real-valued functions defined on $\Omega$, which have a compact support and are infinitely differentiable.
When $u$ is a $C^1$ function, the weak derivative is equal to the usual derivative almost everywhere.
In this case, let us compute the weak derivative. Fix $\varphi\in C^{\infty}_0(\Omega)$ and define $u(x)=(x-1)^+$. We should have $\int_0^2u'(x)\varphi(x)dx=\int_0^1(x-1)^+\varphi'(x)dx+\int_1^2(x-1)\varphi'(x)dx=-\int_1^2\varphi(x)dx,$ so the weak derivative is almost-everywhere equal to the characteristic function of $(1,2)$.
Indicator functions have a weak derivative since these one are locally integrable functions. For example, with $u=\chi_{(0,1)}$, we have $\int_0^2u(x)\varphi'(x)dx=\int_0^1\varphi'(x)dx=\varphi(1)-\varphi(0)=\delta_1(\varphi)-\delta_0\left(\varphi\right) ,$ so the weak derivative of $u$ is $\delta_0-\delta_1$ (which is not locally integrable, but it is still a distribution).