I think the idea is that life is a path, and each of us walk our own path. When we talk about "people from all walks of life", we mean many different people with many different experiences.

I agree with Peachey.  And, yes, it is a very common expression.. at least here where I live. 

Hi labisettrice!

"people from all walks of life" just means "all different kinds of people".
You could use it in a sentence like this:

People from all walks of life come to italki to learn foreign languages. 

I took a train all the way across Europe, and I met many people from all walks of life.

I hope these examples help you.

Un saluto dalla Hong Kong!