就是 has multiple uses: (1) emphasis - 他就是老板 (He IS the boss); (2) concession - 就是...也... (even if...still...); (3) 'the thing is' - 就是有点贵 (it's just a bit expensive).
usage
In casual speech, 就是 is a common filler, like 'well' or 'you know': 就是嘛! (Exactly! / That's what I'm saying!)
