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