This is the 1st most frequent Arabic word.
في
"In" or "inside."
Here, 'في' describes the time when the action was performed, corresponding to 'in' as related to time in English.
فتحت الباب في الصباح الباكر.
I opened the door in the early morning.
In this example, 'في' serves as part of an expression 'في رأيي' meaning 'in my opinion', showing its use in idiomatic structures.
في رأيي، هذا الكتاب مثير للاهتمام.
In my opinion, this book is interesting.
In this sentence, 'في' is used to indicate the location where the subject lives, equivalent to 'in' in English.