Traditionally, rockets have utilized one of two general types of fuel - either solid fuel or liquid fuel. Each has its advantages and disadvantages. The former is very stable, allowing it to be stored for long periods of time in a ready rocket motor without damage or harmful environmental contamination. However, it is much more difficult to control a solid-fuel motor's thrust; practically, you're limited to deciding when to turn it off. Also, solid fuels typically can't contain as much energy as liquid fuels can.
A liquid-fuel rocket can vary its thrust by changing the rate of flow of its propellant and oxidizer, making it more suitable for more complex missions. However, liquid rocket fuel tends to be extremely volatile and/or corrosive; the more powerful types are generally toxic as all heck. This means that handling it is dangerous, and that generally it can only be loaded into the vehicle just before flight.
Efforts are underway to create a third option - that of gelled fuel. As the name suggests, such a fuel would consist of a liquid rocket fuel somehow modified or combined with another substance in order to create a gel. When in storage, the gel would behave more like solid rocket fuel, being much more easily contained, stored and handled than liquids. However, when in use, the gel would behave like a liquid fuel - being capable of variable flow to the engine, liquefying only in the combustion chamber itself.
One industry with a great deal of experience with gel handling, the food preparation industry, has been enlisted to work with more traditional aerospace engineering to learn more about how gels behave in the critical and highly specific environment of a rocket engine.