The Laplace transform takes a function of time and transforms it to a function of a complex variable $$s$$. Because the transform is invertible, no information is lost and it is reasonable to think of a function $$f(t)$$ and its Laplace transform $$F(s)$$ as two views of the same phenomenon. Each view has its uses and some features of the phenomenon are easier to understand in one view or the other.
We can use the Laplace transform to transform a linear time invariant system from the time domain to the $$s$$-domain. This leads to the system function $$G(s)$$ for the system –this is the same system function used in the Nyquist criterion for stability.