I heard of the expression "meat and potatoes business", and when I was explained what it means, I was told is that it referred to a business with no fancy products, just simple products for simple clientèle. Is this a fair description of "meat and potatoes" as an activity?
Yes. It means that your target group are people whose needs you are able to identify and satisfy, in a way as simple and effective as a dish of meat and potatoes. The latter satisfies your hunger completely, so you don't need anything else.
Merriam Webster defines "meat-and-potatoes" as "fundamental, basic. I would say a "meat and potatoes" business concentrates on producing a basic product that is a proven seller to a wide segment of the market. Such a business would not be one to take risks on an untried product or cater to fringe or specialty markets.