What is one name server goes down?

[Matthew Connor]

Hi all. I am new to BIND and I have a probably simple question. I have
two name servers, both configured to be masters: NS1 and NS2. If one of
the name servers goes down, what will be the effect on any resolvers?
Will the query failover to the functioning name server or will
resolution fail? If resolution will fail, is configuring what is known
as a "slave" name server the answer? Thanks for any help!

Matthew