Here is part of the Tips and Notes:
Adjectives follow gender distinction: masculine, feminine, neuter. They decline as nouns and can be divided into groups according to the endings of the nominative singular.
I have a suggestion.
Maybe it would be better to rewrite the second sentence as follows: The endings of adjectives change (decline) in the same way as the noun endings change. See the declension table further down for the adjective endings. Adjectives can be divided into groups based on the endings of the nominative singular.
This is a little more wordy, but easier to understand for someone who is not an expert in linguistics.
I'm assuming that the notes for the Genitive and Accusative skills explain what a declension is and that the basic form of a noun is the nominative singular.
I feel like the word 'decline' couldn't mean something more than 'ending change', and I'm sure not just experts in linguistics know that. (I wrote these tips and notes and I wouldn't call myself a linguistic expert. :P)
However, if you feel like it is so unclear that no one would understand what it refers to, even if they looked at the declension tables right below, we'll consider rephrasing it.