High entropy is a high level of disorganization in a physical system, such as molecules in a gas. Entropy is a quantity used by physicists to describe how organized systems are on the atomic and subatomic level.
Organization in this case refers to the number of possible ways that particles can be arranged within the confines of a physical system. Particles that are jumbled together can arrive at that state a multitude of different ways, while particles that are arranged in a neat stack can only arrive at that state in one particular way.
Entropy is tied to the amount of energy in a system because more energetic particles tend to be disarrayed as they move around. Therefore, systems with high entropy also tend to have more energy than systems with less entropy. For example, solid ice water has particles arranged in neat hexagons, but when energy is added to the ice in the form of heat, the hexagon structures break down into the constantly shifting molecular bonds of liquid water.