As already mentioned "aquel" (aquellos, aquella, aquellas) could be an adjective, but it can also be a noun. Pretty much the same "that" can do in English. That house is big. Aquella casa es grande. ("That" and "aquella" are adjectives). I do not want to talk about that. No quiero hablar de aquello. ("That" and "aquello" are nouns). "That" and "aquel" refer to something that is far from the speaker, but do not fall into the temptation to translate it as "that over there". Since it will not always fit properly in the translation, as you can see comparing the last example... "I do not want to talk about that over there" would be a very bad translation of it. I hope this helps.

aquel(aquellos for plural) and aquella(aquellas for plural) are used as adjectives to say that except when they are farther away so when you say "aquel gato" for example, it means "that cat over there"