Note that cardinal numbers (un, cinq, dix, cent...) never change.

I think what they're trying to say here is that cardinal numbers do not change* in relation the the noun they qualify, whereas ordinal numbers do.

Cardinal: quatre garçons, quatre filles.

Ordinal: le premier ministre, la première fois, les premiers pas, les premières études.

*un/une is slighly different since this it also doubles as the indefinite article:

un jour, une journée

Hope this helps,

Tom