The Earth rotates on a tilted axis (23.5 degrees). This means that during summer in the Northern hemisphere, the North is tilted toward the sun, meaning it is closer to the sun (although this isn't the reason its warmer) and it spends more time in the sun (this is why). As Earth continues to orbit throughout the year, the same side that was tilted towards the sun will remain titled that same direction, however now it is further round, and now the Southern Hemisphere is the one getting more sunlight.
Place a fist in front of you. Tilt your palm toward this fist (in between your face and the fist and fingers close to the fist). Make your titled hand orbit around your fist but keep the fingers facing the same direction always (our rotation is on this axis so it does not change its direction of tilt) as your hand moves around your fist, the fingers are further away when that hand is opposite your fist, but when it is between you and your fist they are close again.
Round about way of thinking about it, that would have been much easier in person, sorry.