Polymers are macromolecules (usually organic) which are formed by the bonding of smaller molecules called monomers.
It has been increasingly used, among other purposes, for gardening. In this case the polymer used is a potassium polyacrylate, unlike the sodium polyacrylate used in the manufacture of nappies and sanitary towels. It comes in the form of solid granules and when such small crystals are dry, they swell to absorb water, turning into a sort of jelly.
Its main feature is that it can absorb between 200 to 400 times its dry volume, which is one of the main reasons for its use with plants. In landscaping, it is often mixed in small doses with the substrate (between 5 and 10 %) in order to increase the capacity for water retention.
As the substrate becomes rigid, the polymer will slowly release a little water and help to irrigate the area, which can be very useful in guerrilla growing or on a terrace, where frequent and regular watering cannot be guaranteed.
Note also that these are non-toxic and last for about two years, after which they break down.