A fluid parcel is an infinitesimal, indivisible piece of fluid - effectively a very small fluid parcel of fixed mass.
The total (or material or substantive or advective or Lagrangian) derivative is the rate of change of a property of the fluid parcel.
Consider the temperature, $T$, of a parcel. The change in $T$ is given by:
$$ \delta T = \frac{\partial T}{\partial t} \delta t + \frac{\partial T}{\partial x}\delta x + \frac{\partial T}{\partial y}\delta y + \frac{\partial T}{\partial z}\delta z$$
So, the change in $T$ in time $\delta t$ is:
$$ \frac{\delta T}{\delta t} = \frac{\partial T}{\partial t} + \frac{\partial T}{\partial x} \frac{\delta x}{\delta t} + \frac{\partial T}{\partial y} \frac{\delta y}{\delta t} + \frac{\partial T}{\partial z} \frac{\delta z}{\delta t}$$
and so the total derivative is given by:
$$
\begin{align*}
\frac{DT}{dt} &= \frac{\partial T}{\partial t} + u \frac{\partial T}{\partial x} + v \frac{\partial T}{\partial y} + w \frac{\partial T}{\partial z} \\
&= \frac{\partial T}{\partial t} + (\mathbf{v} \cdot \nabla ) T
\end{align*}
$$
