This is the 3435th most frequent Arabic word.
لي
"لي" means "for me" or "mine."
The word 'لي' is used here to indicate possession, meaning 'mine'.
هذا الكتاب لي.
This book is mine.
The word 'لي' is part of the word 'ليلاً', which means 'tonight', showing the time as a direct object's descriptor.
أريد أن تحدثني ليلاً.
I want you to talk to me tonight.
The word 'لي' is used here to indicate that the action is done for my benefit, equivalent to 'for me'.