why they need my card account number? Is that will charge my account?

My phone is iOS 8.3, Yesterday I want to download a apps name data manager to control my data,the page only show get,when I press get the apps let me key the password or Touch ID,after this step they let me key my card number ,if I don't key the number apps store don't let me download,after few time I try also cannot,then when I fill up the card number, can download already. is that will charge my account? Why the apps didt show need any fee?please help me.