Hi and welcome.
The manual available on the wiki holds the answer, did you search/read the wiki?
Anyway, normally you would not fill out anything in this field, unless you have a corporate DNS setup and you know what you are doing. Simply leave it blank. This is a default feature, not a contrib feature.
Why are you installing SME 8.0 whilst version 8.2 is the available stable version? Why are you installing 8.0 whilst version 9.0 (and 9.1 around the corner) is available?
Just curious, would you happen to know user ahmedsa? If so, maybe you two can work together?