A simplified explanation could be that behöver refers to a need that comes from the person or situation in question, whereas måste comes from outside.
Läraren säger att du behöver studera mera 'The teacher says that you need to study more' – this implies that it would be good for you to study more.
Läraren säger att du måste studera mera 'The teacher says that you must/have to study more' – this implies that the teacher orders you to study more.
In practice it's always hard to distinguish what's really meant with this kind of verb. There's a lot of room for interpretation and a situation can usually be viewed from many angles. But it seems to me that in Swedish, we do sometimes distinguish between these verbs, whereas in English, the distinction has basically been lost. So all of 'need to', 'have to', and 'must' are accepted answers here. (One of them was missing but I've added it now).
I think the expression tends to be used in more abstract texts. https://www.theguardian.com/news/datablog/2013/jan/10/how-much-water-food-production-waste