What is the meaning of "The backbone of something"? [closed]

In my university, they provided me with some business articles to read. However, I finished with this idiom (the backbone of...) and I didn't get it very well.

The context was the following one:

Our corporation started to expand our business in the year 2001 and the backbone of our business was to export materials to different places all over South America...

I was doing small research about it and I found some information in Google, for example in MacMillan Dictionary:

https://www.macmillandictionary.com/dictionary/british/the-backbone-of-something

And I found that it means: "the part of something that makes it successful or strong"

Personally, I wasn't sure if it was an idiom or what exactly, also, my teacher is not fluent in English and she couldn't provide us a good explanation about it. I'd like to get a better definition or why it's connected to a back? Thanks for your help.

The spine -- the backbone -- acts as the fundamental structural support in keeping humans upright and houses the all-important spinal cord. Without it, we would be useless. When something is referred to as the "backbone" of something else, it is usually the most vital thing about it, and necessary for function.