I am studying QFT but I need to fill some gaps in my comprehension of special relativity (I didn't study it very well and I know I still misunderstand things in S.R).

In my book it is written:

> 
> " A system is Lorentz invariant if it is symmetric under the Lorentz group"
> 
> 
> 

I would like to clarify a little more this sentence.

What do we **exactly** mean by "symmetric under the lorentz group" ? 

Does that mean that if a given (but any) system is described by **any** equation $(1)$ in a referential $R\_1$ (coordinates $x\_1$), if I do an inertial change of frame to $R\_2$, the new equation will be the same as $(1)$ but with $x\_1$ replaced by $x\_2$ ?

I insist on the "any" in my paragraphs.