meaning - What does "Stable-ish" mean exactly?

It means more or less stable

dictionary.reference.com: -ish

a suffix used to form adjectives from other adjectives, with the sense of “somewhat,” “rather” ( oldish; reddish; sweetish )

You can add -ish to most anything and a (British) native speaker will understand it means more or less of what you added it to.

That goes for time: five thirty-ish; and age: She looks 40ish

In spoken English, you add a slight pause to emphasise it and you can even use the suffix on its own

"Are you OK?"
"yeah... ish"

meaning I am more or less OK