A lot of people commenting on this are assuming that politics means democracy. This quote has nothing to do with democracy. Politics can exist in any social organization be it democracy, republic, socialism, communism, dictatorship, etc. Politics has to do with your interests and forming associations to promote your them. So what Pericles is saying is that if you don't get involved in politics (e.g. vote, or form groups to promote your interests, etc) you are going to be overrun by those that do worry about their interests, if they happen to be opposite yours.