Publication fee is the fee for the maintenance of the journal and for keeping it indexed in various indexing services like Google Scholar, Thomson Reuters, DRJI and many more (Refer to indexing  page for more info).
We only ask for publication fee once the paper is accepted and not before that. Once your paper is accepted, you will be asked to send the copyright form along with the publication fee. You will receive email as well as sms requesting you to submit the fee to proceed further