Non-judgement refers to the act of not judging. To judge means to define a thing or a person as either good or bad. Judging is based on one's opinion and learned experiences.
Being non-judgmental can be good when you meet someone from a different cultural background. Being non-judgemental is recognising that you don't know a person or a situation well enough to form an opinion. Being non-judgemental shows that you are willing to remain open and learn.
However, a person cannot be completely non-judgemental. Judgements are needed to operate and navigate this complex world.
Also, being non-judgemental can easily be misunderstood as not having or withholding an opinion. This becomes a danger when you need to voice your opinion on significant issues and on things that matter to you. For example, when you witness harassment, the harassed continues to be in danger if you choose to remain silent and non-judgemental.
Instead, prepare yourself with many opinions. But, be sure your opinions are informed, humane, compassionate, open and flexible. As you learn and gain experience, you will change. When you change, so will your judgments.