In chemistry and physics, a phase is a physically distinctive form of matter, such as a solid, liquid, gas or plasma.
A phase of matter is characterized by having relatively uniform chemical and physical properties throughout.
A single phase is anything that has same physical and chemical properties.
A 2 phase means it has one property at some parts, but different properties at other parts.
A salt water solution is a single phase. This means all properties like concentration, conductivity etc are same throughout.
Oil water system is 2 phases. This means, one plase has a set of properties, but other phase has a different set of properties.