can someone tell me what is topics and how can use this ??!!!
Hi @Diyaa_Daifi
Are you asking about the “New Topic” button?
If so, a new topic means a new thread, and you can start a new topic if you have a question that you can’t find any answer from the existing topics (the existing threads).
For example, this very thread we are discussing in is a new topic that you have created for a question that neither I have ever seen any one asking, so you have demonstrated a good example of when to start a new topic 
It is also a good practice to start a new topic whenever you have a different question, so that every topic has one very specific focus only.
Cheers,
Raymond