This is the 24th most frequent Spanish word.
muy
"Very."
'Muy' is used as an intensifier to enhance the adjective 'feliz' (happy).
Estoy muy feliz hoy.
I am very happy today.
Here, 'muy' intensifies the adjective 'interesante' (interesting), showing admiration or noticeability.
El libro es muy interesante.
The book is very interesting.
In this context, 'muy' (replaced by 'mucho' because 'calor' is a noun specific structure in expressions of weather) denotes an extreme condition.