Being “at opposition” just means that Mars and the Sun are on opposite sides of the Earth. So Mars is visible all night in the sky.
Which, for those who are having trouble visualising, means that the two planets are making their closest approach to each other. This means a lot when you’re looking at a small planet like Mars - it’s bigger than the Moon but a lot smaller than the Earth, so the only way it looks bright is to be close. Jupiter, on the other hand, is freakin’ huge so if it’s visible at all it’s usually pretty bright (and it’s pretty distant too so the odd month or two either side of opposition doesn’t affect the distance all that much).