What is the physical significance/interpretation of a vanishing Lie Derivative? In my lectures, an isometry of the metric is introduced as follows: 

A flow on a manifold $M$ is a one-parameter family of differomorphisms $\sigma_t:M \to M$. The flow is said to be an isometry if the metric looks the same at each point along a given flow line. 

This definition is very intuitive and makes quite a lot of sense but the mathematical definition is less intuitive: 

Mathematically, this means that an isometry satisfies 
  $$
\mathcal{L}_K (g) = 0 \iff \nabla_\mu K_\nu + \nabla_\nu K_\mu = 0
$$ 
  where $ K^\mu = \frac{dx^\mu}{dt}$ is the vector field on $M$ that is tangent to the flow at every point and $\mathcal{L}_K$ is the Lie Derivative of the metric. 

I understand how the LHS expression is mathematically equivalent to the RHS expression, but I am not so sure about the physical significant of either expression. In particular, I am curious if there is any physical significance or interpretation to the vanishing of a Lie Derivative (of tensors or vectors)? 
 A: Imagine you have a vector field $\xi^\mu$. Imagine you have a coordinate system $x^\mu$ and want to define a new coordinate system
$$
x'^\mu = x^\mu + \varepsilon \xi^\mu
$$
which we can also write as
$$
x^\mu = x'^\mu - \varepsilon \xi^\mu
$$
where $\varepsilon$ is tiny. What is the metric in the $x'^\mu$ frame? Well,
$$
g'(x')_{\mu \nu} = \frac{\partial x^\alpha}{\partial x'^\mu} \frac{\partial x^\beta}{\partial x'^\nu}g(x)_{\alpha \beta}
$$
and
$$
\frac{\partial x^\alpha}{\partial x'^\mu} = \delta^\alpha_\mu - \varepsilon \partial_\mu\xi^\alpha
$$
so, only keeping terms to the first order in $\varepsilon$,
\begin{align*}
g'(x + \varepsilon \xi)_{\mu \nu} &= (\delta^\alpha_\mu - \varepsilon \partial_\mu\xi^\alpha)(\delta^\beta_\nu - \varepsilon \partial_\nu\xi^\beta)g(x)_{\alpha \beta} \\
g'(x)_{\mu\nu} + \varepsilon \xi^\rho \partial_\rho g_{\mu \nu}(x)&= g(x)_{\mu \nu} - \varepsilon \partial_\mu\xi^\alpha g(x)_{\alpha \nu} - \varepsilon \partial_\nu\xi^\beta g(x)_{\mu \beta}
\end{align*}
Therefore, under this infinitesimal diffeomorphism of translation along the vector field, we can see that the change in the metric at point $x$ is
\begin{align*}
\delta g_{\mu \nu} &= \tfrac{1}{\varepsilon}(g'(x)_{\mu \nu} - g(x)_{\mu \nu}) \\
&= -\xi^\rho \partial_\rho g_{\mu \nu} - \partial_\mu\xi^\alpha g_{\alpha \nu} - \partial_\nu\xi^\beta g_{\mu \beta}.
\end{align*}
Next, note that the Lie derivative of the metric along $\xi$ is
\begin{align*}
\mathcal{L}_\xi g_{\mu \nu} &= \nabla_\mu \xi_\nu + \nabla_\nu \xi_\mu \\&= g_{\nu \alpha} \nabla_\mu \xi^\alpha + g_{\beta \mu} \nabla_\nu \xi^\beta \\
&= \big( g_{\nu \alpha} \partial_\mu \xi^\alpha + \Gamma_{\nu \mu \rho} \xi^\rho \big) +\big( g_{\beta \mu} \partial_\nu \xi^\beta + \Gamma_{\mu \nu \rho }\xi^\rho\big) \\
&=  g_{\nu \alpha} \partial_\mu \xi^\alpha + g_{\beta \mu} \partial_\nu \xi^\beta + \big( \Gamma_{\nu \mu \rho}  + \Gamma_{\mu \nu \rho } \big) \xi^\rho \\
&=  g_{\nu \alpha} \partial_\mu \xi^\alpha + g_{\beta \mu} \partial_\nu \xi^\beta + \partial_\rho g_{\mu \nu} \xi^\rho \\
&= -\delta g_{\mu \nu}
\end{align*}
So we have now seen the interpretation: the Lie derivative of the metric along the vector field is the infinitesimal change of the metric under the tiny diffeomorphism of a translation along that vector field.
In fact, this is in general intepretation of the Lie derivative of any tensor, it's just the change in the tensor under the tiny diffeomorphism $x^\mu \mapsto x^\mu + \varepsilon \xi^\mu$.
For a concise introduction, consult section 1.4 of Eric Poisson's book "A Relativist's Toolkit." Lie differentiation is actually a more "primative" operation than covariant differentiation. You don't need a metric or Christoffel symbols to define it. However, it can also be written in terms of covariant derivatives, which is fortunate. When $\xi$ is a "symmetry" of our metric, then the metric doesn't change under this transformation.
A: The geometric/physical meaning is that the components of the metric tensor do not change along the direction of the field $K$. Such a field is called a Killing vector.
An isometry of manifolds is something else and there is no preferred direction. 
I do not know why your lecturer uses this non standard terminology
