Secret ballot is an integral (and almost central) idea to
democracy and
elections. It is the idea that when you
vote, you do so secretly and without fear of
retribution for who/what you vote for. This is vital to
democracy since if everyone knew how you voted then it is possible that they would punish you because of it or threaten you before hand.
This was a problem that existed throughout England since the creation of a Parliament in 1275. These elections started off being only for certain important men and were open (not secret). This didn't seem much of a problem at the time because a wealthy aristocrat tends not to be easily threatened since they hold wealth and power (though it is possible that persecution still happened). However, when the Reform Act 1867 was introduced over 1.5 million adult males were given the vote (most of these were working class men). This meant that a lot of factory workers, land labourers etc. were able to vote.
This had consequences in that these poverty stricken people could be easily threatened by their employers/land lords. Employers could easily sack and replace workers who did not vote the way that they wanted them to and land lords could easily evict people if they did not vote for the preferred candidate. It was often the case that when casting a vote you had to stand on a platform and announce to the official (and the rest of the crowd) who you were voting for. If that wasn't the case then people could at least look over your shoulder while you were voting.
This was changed in the Ballot Act 1872 (brought in by William Gladstone's government) which brought in secret voting (it is also mentioned in the Human Rights Act which has a section stating "to hold free elections at reasonable intervals by secret ballot").
It tends to be the first thing that is abused in a democracy. In states around the world the West often calls for there to be 'free elections' for the people (such as a few African states which claim to be democratic). This is synonymous with the secret ballot. Democracy is no good without it.