This is the 25th most frequent Italian word.
sul
"On the" or "about the" (a contraction of "su" + "il").
Here, 'sul' is a contraction of 'su' (on) and 'il' (the), serving as a prepositional phrase indicating location.
Il libro è sul tavolo.
The book is on the table.
In this context, 'sul' combines 'su' (on/about) and 'il' (the) to form a prepositional phrase referring to the subject of reflection.
Sto riflettendo sul problema.
I am reflecting on the problem.
Here, 'sul' merges 'su' (on/about) and 'il' (the) to introduce the topic of discussion.