A block diagram represents a system as interconnected transformations of signals.

If two systems are connected in series, the output of the first becomes the input of the second. For transfer functions,

$$G_{series}(s)=G_2(s)G_1(s).$$

If two outputs driven by the same input are added in parallel,

$$G_{parallel}(s)=G_1(s)+G_2(s).$$

A summing junction forms algebraic combinations of signals, while a takeoff point sends one signal to several destinations without changing it.

A feedback connection routes a downstream signal back to an earlier summing junction, creating an algebraic loop. Its equivalent input-output transfer function is found by writing equations for the internal signals and eliminating those internal variables.

Block diagrams are not physical schematics. They expose signal flow and let a compound system be reduced systematically to an equivalent relation between chosen inputs and outputs.