If you're overconfident, you're so sure of yourself that you're pushy and obnoxious. It's important to be confident at a job interview, but if you're overconfident, you may come across as downright rude.
There's a fine line between confidence and overconfidence: being confident is attractive and appropriate. When someone seems overconfident, though, they appear to be unrealistically sure of themselves, more arrogant than poised. Overconfident combines the prefix over-, "too much," and confident, from the Latin confidentem, "firmly trusting or self-confident."