what is an incoming mail server

I would guess, if you are seeing this, that you are setting up an 'other' email setting rather than googlemail, icloud or something like that etc.

If this is the case, then you need to contact your supplier of that email system / ISP or take a look at their webpages. They are bound to be able to give you the appropriate settings for the iPhone and they should present it all to you such that you can recognise the various settings and bits of information which you need - including incoming and outgoing email server etc.

The outgoing mail server is a computer somehwhere that accepts and forwards your emails. An Incoming Mail server accepts emails addressed to you and allows you to connect and retrieve them.

Your email provider will inform you of the outgoing and incoming mail servers and you most likely specified a password when you signed up for the email service.