It is easeir to understand this at low freq.
Circuits may exibit more noise current or more noise voltage. Thus input noise depends on the resistance of the source. For low source resistance the noise current will have little effect but noise voltage will be dominant. For high source resistance the noise current will dominate. For example, FET may have small noise with high source resistance because they have near zero DC input current.
So there is some optimum source resistance for which input noise will be minimum. For RF freq it is more complicated because of complex impedance character.