Polymers are macromolecules (usually organic) formed by the association of smaller molecules called monomers.
Among its many applications, their use in gardening is remarkably increasing; in this case, the polymers used are potassium polyacrylate unlike the sodium polyacrylate that is normally used in the manufacture of diapers and sanitary pads. It comes in solid granules (small crystals when dry) which swell when absorbing water, becoming gelatinous.
Their main feature is that they can absorb 200 to 400 times their dry volume, thus being extremely helpful when used with plants. Polymers are usually mixed in small amounts with the substrate - 5/10% - in order to increase the water retention capacity of the soil. As the substrate dries, polymers will slowly release water, reducing the watering needs of plants, what may be particularly useful in balcony or guerrilla crops, where regular irrigation can't be guaranteed.
Polymers are non-toxic and last for about two years, when they finally degrade.