Basically you put in your default email server settings from your email account.
The from address is usually the email address associated with that default email account, though it could be any valid email address. It is effectively the same as when you send an email, the From address.
You don't have to contact them, it is there if you need to send a virus sample to avast, it uses these details to send it. You can also have an email sent to someone if a virus is detected (Alerts section of Program Settings), like if you had children using the computer you could send yourself an email at work say. However, to be able to do that the you need to have entered the SMTP information.
I have to say I have never managed to get that working on my system.
