Hey there, @HamidaSurti.
Allow me to explain unapplied cash payment income to you.
It's to report cash basis income from customer payments that are received but not applied to any sales form. Both QBO and QBD will automatically create a credit for the customer you wish to add a cash payment for, so this could be used at a later time.
Check out, What's Unapplied Cash Payment? for more details.
Please let me know if I can be of more assistance. You can always reach out if you have further questions. Take care and have a good one!