What is an in input?

Answer 1

An input is what the firm uses in order to produce a final good.

So it can be material, like glass to make light bulbs,

but more often these are generally looked at as intermediate goods.

A true input is usually capital (K) or Labor (L) used to produce a quantity (Q) of a good.

so a simple production function can look like this

Q=K+L

meaning that if you had 3 units of K and 5 units of L you would have 8 units to sell.

This type of production function where K and L are added is indicative of inputs being perfect substitutes.