This is the 34th most frequent Hindi word.
आज
"Today"
In this sentence, 'आज' indicates the time when the action of reading took place, specifying today.
आज मैंने एक नई किताब पढ़ी।
Today, I read a new book.
Here, 'आजकल' (rooted from 'आज') conveys the idea of modern times or recent days, extending the usage of the word to reflect a more continuous time frame.
आजकल लोग बहुत व्यस्त रहते हैं।
These days, people remain very busy.
'आज' is used here to refer to the current day (today); it situates the sentence in the context of the present day.