When a
satellite has an
elliptic orbit around a
planet, the perigee is the point in which it achieves the
minimum distance to the planet's center. That minimum distance is also named after the perigee. It is also the point in which the
satellite has the fastest
orbital speed compared to all other points in its orbit.
It is important to know about the perigee when tuning antennas, because the perigee is the least desired point to direct the antenna to. The fast movement of the satellite at that point means it will be accessible in the sky for a rather short time, and the antenna will need constant adjustments.
The apogee is the opposite point to the perigee. The perihelion and perilune are perigees of planets around the Sun and Moon.