What is system input?

First you have to understand what a system is.

sys·tem
noun
1. a set of connected things or parts forming a complex whole, in particular.
2. a set of principles or procedures according to which something is done; an organized scheme or method.
"a multiparty system of government"
synonyms: method, methodology, technique, process, procedure, approach, practice;

When you pick a particular system, the inputs are defined (usually). In math a system is called a function and it is defined as any process that takes in a qualified number and puts out a single value. A square root fails the definition because it puts out two numbers for each qualified input.

A car has a steering system where the inputs are position of the steering wheel, position of the tires, speed of the car, bumps in the road, and maybe some smaller details. Racing is a big part of designing good cars because they test every detail. That is how engineers discovered that steering is affected by driving over a bump. Each of these details is a system input, while the output is a vaguely defined "stability".

Sorry to give you such a vague answer, but you have asked a vague question.