Each Steem community member gets 30 votes to vote for witnesses. The witnesses maintain nodes that produce the Steem blockchain which pumps out all our posts etc.....it goes deeper but that should suffice for now.
A. you don't do anything and by de facto you leave Justin SUN in full control of the Steem Blockchain and you join the darkside
B. You vote for J. Sun's witnesses and you join the darkside
or
C. You use your 30 alloted witness votes to vote for the original Steem witnesses and not J. Sun's.

Any more questions and have you voted?