Hey Ern Jun,
So everything, all nouns in Spanish have a gender (feminine/masculine) whether they are alive things or inanimate objects, including abstract concepts! What the sentence above is saying is that there is no ''rule'' to ''predict/assign'' a gender to something. It is pretty arbitrary and most of the time you just have to rely on practice and memory. Except of course, in the case of living things, where the grammatical gender is the same as the biological gender.
Living things:
EL gato, EL perro, EL hombre. LA gata, LA perra, LA mujer.
inanimate objects:
EL libro, EL papel, EL celular. LA mesa, LA fresa, LA cama.
abstract concepts:
EL amor, EL capitalismo. LA idea, LA teoria, LA gravedad.
Hope that helps. :)