RequestParameter in Ampscript and how it is used?

We have a html form and from this form we fetch email address.

For example, the form input field looks like this,

<input type="text" name="EmailAddress">

Then there is the amp-script code to fetch information from the form on submit

set @EmailAddress = Replace(RequestParameter("EmailAddress")," ","+") if empty(@EmailAddress) then set @EmailAddress = Replace(RequestParameter("email")," ","+") endif

This is built by someone in our team in the past and I am trying to understand what exactly the ampScript does here with replace? I am totally not able to understand this part. I only know that EmailAddress Filed is being fetched form input. Please can anyone help me understand this . Thanks