This phrase is very much context driven, but generally refers to the material's behaviour at standard conditions ie room temperature, 1 atmosphere pressure, oxygen bearing atmosphere.
A usefully vague term, used to encompass, amongst other things
the state of matter; solid, liquid or gas
the material's reactivity towards other materials
preferred oxidation state/s
molecular shape and bond orientation
bond length
coordination number
preferred types of bonds to form eg metallic, ionic, covalent