Here are more detailed explanations:
Name: This is the displayed name of the web. If in doubt, use your full name.
Address: This is how the web name will appear in URLs. It should be lowercase and without spaces. If in doubt, use the Name but take out the spaces and make it lowercase.
Markup: This is the syntax used for writing wiki pages. The default is what is used on the main nLab so you probably don't want to change this.
Color: Whatever you like best!
Safe mode: The main Lab doesn't need this, so you probably don't either.
Brackets only: If you want links to work as in the main Lab, use this.
Count pages: If selected, a count of the number of pages on the web will be displayed at the foot of each page.
Allow uploads: If you want to upload files to your personal web rather than the main nLab web; increase the size also if you want to upload large PDF files and the like.
Stylesheet tweaks: If you want query boxes and the like, then copy these from the main Lab.
Password: Leave this blank if you want your web to be edited by anybody, like the main Lab is.
Publish: If you have a password, then you need this if you want others to read it. If you don’t have a password this option should not be selected. It is redundant and can cause confusion with wikilinks.
The most important ones to choose are the last two: Do you want your web to be open like the main Lab, readable by all but with restricted edits, or completely private? Colour is not important, and the rest we can probably guess.