I know that "all walks of life" is an idomatic sentence but what it really means? it is very common in spoken language ?
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.
"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!