The point in its orbit at which a planet is furthest from the Sun.
The orbits of the planets are not perfect circles: they are ellipses. And the Sun is not at the centre of these ellipses, but slightly to one side (at what is called the focus of the ellipse). For this reason the distance of a planet from the Sun varies: the aphelion is the furthest point, the perihelion is the closest.
In the case of the Earth, the difference between the aphelion and perihelion is about five million kilometres. This is about one-thirtieth of its average distance from the Sun, which is about 150 million kilometres. This does not have a significant effect on the climate, but the visible size of the Sun is slightly smaller at aphelion compared with perihelion. (The seasons are not a consequence of aphelion and perihelion: they are a result of the tilt of the Earth’s orbit.)
The date of the aphelion of the Earth varies slightly each year, but in 2018 it was on 6th July.