With most adjectives you can cut off the ending and replace it with -tas, -tatis, f. to get a third declension abstract noun related to the adjective and usually translated with a word ending in "ty" in English.
Example: Humanus, -a, -um, which means human (the adjective). Cut off the ending to get the stem human-. Add the suffix -tas to get humanitas, humanitatis, f., which means "humanness", or, "humanity".
Hope this helps.