Neuter plurals end in -a in the nominative and accusative, and a neuter plural adjective with no noun means "things": bona "good things", omnia "everything".
The neuter plural nominative and accusative always end in -a: bona, omnia, ista, quae, corpora, opera (from opus). Do not take such a word for a feminine singular: in Deorum opera providentiae plena sunt, the plural sunt and plena show that opera means "the works".
A neuter adjective alone stands for "thing(s)": bona ac mala "good and bad things", omnia "all things, everything", humana "human affairs", alba et nigra "white and black"; in the singular, nihil grave "nothing grievous", bonum "a good".
A masculine adjective or participle alone stands for people: boni "good men", actuosi "busy people", irascentium "of angry people".
Neuter pronouns work the same way: ista omnia "all these things", haec "these things", quae "(the things) which".