This map shows the distribution of water across the martian surface. Precisely, it does that by measuring how much hydrogen atoms are in the ground. The hydrogen could be bound in water molecules, but also in other chemicals. The map shows the water-equivalent amount of hydrogen, i.e. how much water would there be in the martian surface if it was all bound in water molecules.
The data is based on the MONS (Mars Odyssey Neutron Spectrometer) instrument which is part of the GRS (Gamma Ray Spectrometer) instrument aboard the Mars Odyssey orbiter spacecraft. link
If you wanna learn more about neutron spectrometers, check this article:
https://www.nasa.gov/solar-system/moon/wheres-the-water-two-resource-hunting-tools-for-the-moons-surface/
The Neutron Spectrometer System
Sensing the amount of hydrogen in the subsurface is the job of the Neutron Spectrometer System, or NSS. It can measure the total volume present, up to three feet below the surface. NSS works by measuring changes in the number and energy of particles called neutrons that are always coming from the Moon. When these tiny particles strike something that’s about their size – like a hydrogen atom – they lose a lot of their energy. That’s a change that NSS can detect and use to infer the presence of hydrogen.