Mini-EUSO (Multiwavelength Imaging New Instrument for the Extreme Universe Space Observatory, known as UV atmosphere in the Russian Space Program) is the first detector of the JEM-EUSO program to observe the Earth from the International Space Station (ISS) and to validate from there the observational principle of a space-based detector for UHECR measurements. Mini-EUSO is a telescope operating in the near UV range, mainly between 290 - 430 nm, with a square Focal Surface (FS) corresponding to a Field of View (FoV) of ∼ 44◦. Its spatial resolution at ground level is ∼ 6.3 km. Mini-EUSO was launched with the uncrewed Soyuz MS-14. The first observations, from the nadir-facing UV transparent window in the Russian Zvezda module, took place on October 7, 2019. The detector size (37 × 37 × 62 cm3) was mainly constrained by the window size. The detector is usually installed during onboard night-time a couple of times per month, approximately at 18:30 UTC with operations lasting about 12 hours. So far, 139 sessions of data acquisition have been performed. Data are stored locally on USB solid state disks. After each data-taking session ∼10% of stored data, usually corresponding to the beginning and the end of each session, are copied and transmitted to ground to verify the correct functioning of the instrument. Till now, the data of the first full 44 sessions returned to Earth and is being analysed. The Mini-EUSO FS, or Photon Detector Module (PDM), consists of a square matrix of 36 Multi-Anode Photomultiplier Tubes (MAPMTs). Each MAPMT consists of 8 × 8 pixels. A group of 2 × 2 MAPMTs forms an Elementary Cell (EC). In total there are 2304 channels. Each EC has an independent high voltage power supply (HVPS) and board connecting four MAPMTs. The HVPS system, based on a Cockroft-Walton circuit, has an internal safety mechanism which operates either reducing the collection efficiency or the gain of the MAPMTs when particularly bright signals occurr. The optics are based on two 25 cm diameter Fresnel lenses with a point spread function of 1.2 pixels. UV bandpass filters are glued in front of the MAPMTs. The system has a single photon-counting capability with a double pulse resolution of ∼ 6 ns. Photon counts are summed in Gate Time Units (GTUs) of 2.5 μs. The PDM Data Processor stores the 2.5 μs GTU data stream (D1) in a running buffer on which runs the trigger code. Sums of 128 frames (320 µs, D2) are continuously calculated and stored in another buffer where a trigger algorithm, at this time scale, is running. Similarly, sums on 128 D2 frames (40.96 ms, D3) are calculated in real time and continuously stored. Every 5.24 s, 128 packets of D3 data, up to 4 D2 packets and up to 4 D1 packets (if triggers were present) are sent to the storage (SSDs). Mini-EUSO measured the terrestrial UV background with unparalleled precision. The fraction of time in which the atmospheric UV light intensity allows the UHECR observation from space is ∼18%, compatible with the expectations from simulations conducted at the time of the JEM-EUSO studies. The trigger rate on spurious events remains within the requirements in nominal background conditions. It proved effective in detecting Short Light Transients (SLT), demonstrating indirectly that the JEM-EUSO technology can detect UHECRs from space as they show similarities in terms of light profile, intensity, duration and pixel pattern on the FS of Mini-EUSO, even though all these characteristics do not match at the same time for a single event and are not mistaken for real EAS-induced signals. The ability of Mini-EUSO to detect and study atmospheric phenomena like ELVES, and the ones linked to SLTs is unique and beyond the capabilities of any other atmospheric detector. That makes a space-based detector for UHECRs a unique instrument for the atmospheric science field. Mini-EUSO demonstrated to be the first experiment from space to perform a systematic study of meteor light curves and flux in a wide range of magnitudes.