Over trading isn't just opening too many trades as some traders can be trading on different trade and will still be able to make profits. Opening different trade at the same time isn't a bad way of trading but newbie shouldn't do it. If you don't have enough experience you can mess things up finish one trade before opening another. This is how I started to trade but later I could multitask and it is still going well for me as I haven't had any problems.
Overtrading for me will be trading when you're exhausted from the days work or from trading already but you still want to trade more and it could be for reasons that aren't disciplined as chasing a loss or you are just being greedy. Every trader should know where their limits are so they don't overwork their brains and it begins to make them to be out of control and making mistakes.