Voting is compulsory in Australia for most adult citizens who are not serving a prison sentence of more than three years. Many countries do not have compulsory voting; such countries include the United Kingdom, the United States of America and New Zealand. There are many arguments for and against compulsory voting. If you look up compulsory voting on Wikipedia there is a list of the countries that have compulsory voting (apparently only 10 countries enforce it), and an interesting summary of the usual arguments for and against this practice.
We have seen recently a low participation rate in certain demographics for the ‘Brexit’ vote, followed by members of those same demographics (for instance young people) complaining about the outcome. We have also seen a high participation of previously uninterested voters in the USA supporting candidates like Donald Trump and Bernie Sanders. The political parties in the USA have struggled to measure and therefore respond to a group of people that were previously considered to be unimportant because they did not vote.
In practice compulsory voting means attending a polling place and having your name marked, or voting by mail. If you make a mistake on your ballot, whether intentionally or accidentally, your vote will not count but you also will not be fined. If you make a mistake filling out the form you can ask for another ballot paper.