"Bipolar 1 mixed" means you experience mania, depression and mixed episodes. Mixed episodes are times when both manic and depressive symptoms appear at once. You might, for example, feel extremely sad (depressive) but have severe racing thoughts (manic or hypomanic). You might feel the intense fatigue and worthlessness that are depression symptoms, but at the same time talk a blue streak not allowing others to interrupt, perhaps jumping from one topic to another unrelated topic so that what you're saying is hard for others to follow.
These are just a couple of examples of what can happen in a mixed episode. Mania/hypomania and depression each have so many symptoms that many, many combinations are possible in a mixed episode.
I hope this is helpful!