Hola Johan
Countable nouns are those we can count in units, for example:
un pájaro, dos pájaros,... (a bird, two birds,...)
un hombre, dos hombres,...(a man, two men,...)
tres casas, cuatro casas,...(three houses, four houses,...)
Uncountable nouns are those that we cannot count in units, for example:
un dinero, dos dineros,... (a money, two moneys ?????)
una felicidad, dos felicidades,... (a happiness, two happiness,????)
This is what we are referring to in the lesson.
Hope it helps,
Inma