These gel-like particles are usually sold as dry crystals to mix with potting soil. They go by the names Watersorb, Soil Moist Terra-Sorb, and others. The actual compound is polyacrylamide, a manufactured polymer that absorbs water, then slowly releases it back into the soil as the soil becomes drier. It's pretty much the same stuff put in disposable diapers to absorb extra moisture. It takes only about a teaspoon of the dry polymer to amend the soil in a 6-inch pot.
Industry sources claim that it lasts for 7-9 years in the soil before it breaks down into carbon dioxide, water, and nitrogen. The gels are a bit controversial for use in ground beds, but most research shows that they will reduce moisture stress and frequency of watering for container-grown plants. That can be a real bonus during those hot, dry summer days when the pots dry out almost as fast as you water them.