A geostationary orbit is a circular orbit where a satellite travels at an altitude of about 35786 km above the earth equator, following the direction of the earth's rotation. At this height the satellite orbital period is equal to the earth's rotational period and thus, to ground observers, the satellite appears motionless, at a fixed position in the sky. The MTG-S satellite has a fixed longitude of about 0 degrees.
The SENTINEL-4 mission is innovative in that it is the first time that a satellite, focused on the analysis of the Earth's atmosphere chemical composition, is positioned on a geostationary orbit.
Indeed all heritage instruments, before SENTINEL-4, have always been flown on Low Earth Orbit Polar satellites at an altitude of about 800 km above the Earth, satellites in geostationary orbit usually hosting instruments dedicated only to meteorology to ensure constant imaging.