Adjectives used with intransitive verbs in lieu of adverbs

That would be a predicate adjective. Instead of an adverb describing how the subject is doing the verb, the predicative adjective simply describes the subject, thanks to the help of a "linking verb" as this website so cutely puts it:

http://www.grammar-monster.com/glossary/predicate_adjectives.htm

Note that this phenomenon can only occur with verbs that are sort of 'modifications' of "to be". So:

I feel good = My body is well
Those shoes look good = Those shoes are good based on their looks (as opposed to their smell, perhaps).
Rest easy = Be easy (wowee!)

Hope that helps!