The 「の」 particle in this usage essentially replaces the noun and takes over the role as a noun itself. We can essentially treat adjectives and verbs just like nouns by adding the 「の」 particle to it. The particle then becomes a generic noun, which we can treat just like a regular noun.
(1) 白いのは、かわいい。- Thing that is white is cute.
(2) 授業に行くのを忘れた。- Forgot the event of going to class.
Now we can use the direct object, topic, and identifier particle with verbs and adjectives. We don't necessarily have to use the 「の」 particle here. We can use the noun 「物」, which is a generic object or 「こと」 for a generic event. For example, we can also say:
(1) 白い物は、かわいい。- Thing that is white is cute.
(2) 授業に行くことを忘れた。- Forgot the thing of going to class.
However, the 「の」 particle is very useful in that you don't have to specify a particular noun. In the next examples, the 「の」 particle is not replacing any particular noun, it just allows us to modify verb and adjective clauses like noun clauses. The subordinate clauses are highlighted.
(1) 毎日勉強するのは大変。 - The thing of studying every day is tough.
(2) 毎日同じ物を食べるのは、面白くない。- It's not interesting to eat same thing every day.