In its most basic form scalability is how something can change in size or scale. However, the definition is not limited to this, and can scale down to a business specific (see what I did there?). For the world of money and products, scalability is most commonly a term that refers to how a company is able to grow in order to meet increased demand and support its own growth.
Commonly, industries will invest in ERP software to help facilitate the changes needed for a company to grow into its potential, automating and managing increased customer demand, product inventory, onboarding and hiring procedures, procurement, and more. All of these are crucial aspects of scalability, as when a business begins the scalability process is starts a race against demand and time, and should the company fall behind, it will be left without the ability to meet customer and company demands, leading to, well, negative results.
The other issue is cost. Scaling up costs money. Money for new employees, money for new machines, money for new infrastructure, money for all the overtime you are going to have to pay, etc. If a business is able to scale efficiently, it is crucial that the increase in revenue gain from scaling up would outpace the increased expenses.
The true measure of how far a company will go is how able they are to scale. Without the ability to keep up with competitors, companies will grow stagnant in their ability to be competitive in the market, and dry up like a fish left on the beach.
Here is a quick example: if we have a company that produces the stuff on the sides of fish that are used to make a tool used to measure weights of things, and the demand for the tool suddenly skyrockets, then the company must increase production, as scales scale with scales.