A property is an attribute of a concrete object or abstract entity. These are the features, qualities, or characteristics that something has, or is described as having. A red apple has the property of redness. A heavy rock has mass. A triangle has three sides. A promise may have reliability. A rule may have fairness. Properties help answer the question: what is this thing like?
Properties apply to both concrete objects and abstract entities. A concrete object is a material thing that exists in reality, and its properties can often be observed, measured, or tested. These map to empirical ideas. An abstract entity is a conceptual thing held in the mind, and its properties are usually understood through reason, comparison, definition, or framework. These map to rational ideas. In this way, properties help the mind move from real things to organized descriptions.
A tree may have height, age, species, location, and health. A star may have mass, brightness, temperature, and chemical composition. A number may have evenness, oddness, primeness, or value. A framework may have coherence, scope, assumptions, and explanatory power. Some properties are easy to observe. Others require interpretation. Some belong to things in the world. Others belong to ideas of the mind.