In the Lagrangian description you describe the motion of material fluid cells. In fluid mechanics it's convenient to simply use the positions ##\vec{x}_0## of your fluid cells at initial time ##t=0## to label each fluid element. Then the motion of the fluid element which was at ##\vec{x}_0## at time ##t=0## at time ##t## is at ##\vec{x}(t,\vec{x}_0)##. The velocity of this fluid element is of course simply ##\vec{v}_0(t,\vec{x}_0) = \partial_t \vec{x}(t,\vec{x}_0)##, where the time derivative is taken at fixed ##\vec{x}_0##, i.e., for the fixed fluid element which was at ##\vec{x}_0## at time ##t_0##. The same is true for the acceleration of this material fluid element, ##\vec{a}_0(t,\vec{x}_0)=\partial_t \vec{v}_0(t,\vec{x}_0)##.
The Eulerian description is a field description, i.e., ##\vec{x}## labels a fixed point in space, and you describe the fluid motion by the velocity field ##\vec{v}(t,\vec{x})##, which tells you the velocity of the fluid element which is at ##\vec{x}## at time ##t##.
Now obviously ##\vec{v}_0(t,\vec{x}_0)=\vec{v}(t,\vec{x}(t,\vec{x}_0))## and thus
$$\vec{a}(t,\vec{x}_0) =\partial_t \vec{v}_0 = \partial_t \vec{v} + [\partial_t \vec{x}(t,\vec{x}_0) \cdot \vec{\nabla}] \vec{v} = \partial_t \vec{v} + (\vec{v} \cdot \vec{\nabla}) \vec{v}.$$
This defines the "material time derivative", i.e., for any quantity ##f(t,\vec{x})## described in the Eulerian description the time derivative corresponding to a fixed material fluid element is
$$\mathrm{D}_t f(t,\vec{x}) = \partial_t f(t,\vec{x}) + (\vec{v} \cdot \vec{\nabla}) f(t,\vec{x}).$$