Not all the water in the soil is useful for the plant. Some of it drains by gravity and is gone; another is so tightly retained by the particles that the root cannot extract it. What remains in the middle is the plant available water.
It is expressed in millimeters—the same millimeters as rain—up to a given depth, usually one meter. A soil with 180 mm of plant available water at one meter can sustain the crop during a dry season while another with 90 mm cannot.
It depends above all on the texture and effective depth. A deep loamy soil stores a lot; a sandy one, little, because it drains; a clayey one retains a lot but delivers with difficulty. A calcareous hardpan at 60 cm cuts the profile and with it the reserve.
It is the data that explains why two properties with the same rain perform differently: the rain is what comes in, the plant available water is what the soil can save for when it doesn't rain.