Unobtrusive describes something — or someone — that is there but does not push itself at you. It does not hide; it simply does not demand your attention. This is nearly always a compliment in English, and that is the part students miss. An unobtrusive waiter fills your glass and disappears, and you had a lovely evening partly because of that. Unobtrusive lighting, unobtrusive music, an unobtrusive repair to an old wall, an unobtrusive security camera — in each case the praise is that it does its job without interrupting anything. With people, unobtrusive suggests good manners: quiet, tactful, not making the moment about themselves. Compare a few neighbours: an obtrusive one is always in your face; an unobtrusive one is around, and you never feel crowded. It is a formal, mostly written word, and in speech people say 'you hardly notice it' or 'it doesn't get in the way'. Its usual partner is 'discreet', which is very close but adds the idea of keeping something secret; unobtrusive is only about not standing out.