[subject] + 很 + [adjective]
很 sits in front of an adjective and holds the slot English fills with "is". Its dictionary meaning is "very", but in this position it usually means almost nothing.
Translating that as "I am very busy" overstates it. Unstressed 很 is a rhythm word: the adjective needs something before it, and 很 is what goes there.
Stress it. In speech a stressed hěn carries the full degree, and the difference is audible.
In writing there is no way to show the stress, so a speaker who really means "very" reaches for a different word.
不 takes the slot instead. wǒ我bù hěn不很lěi累 is not the natural form; say 我不累.
A handful of verbs about feeling and wanting take 很 as if they were adjectives: 喜欢, 想, 爱, 希望.
Ordinary action verbs refuse it – there is no 很吃 or 很走.