If you're ignorant of them, you can honestly say "I declare that i know not of any" etc etc.
And as for the 2nd part of the question, you've asked that the people there witness your union as a couple, that you are willingley entering a legal partnership known as marriage.
Lawful impediments would be: one or both partners are already married (bigamy); they are related (as brother and sister for example); they are of the same sex (go get a civil partnership); one or other is underage.