It'll turn on or off that little envelope icon under your name over there --->
That icon along with the "PM" icon only shows up for people who are logged in. If someone wants to contact you, they can "PM" you (private message), which uses the internal mail-like system in this board. Or they can use the email icon, which will send their message to you via your registered email address in your account here. The sender will not get to see your email address, unless you choose to respond to them via your personal email system.