From electowiki

Abstention is when a voter doesn't use their vote to indicate a preference or otherwise affect the outcome. It is commonly seen in legislative voting.

A related concept is equal ranking/rating, in which a voter indicates no preference between certain options. In most voting methods, such a voter has no influence in determining which of those candidates wins, though notable exceptions include any voting method involving quotas, such as Category:Largest remainder-reducing voting methods and Category:Graded Bucklin methods. Also see Independence of Irrelevant Ballots.

Abstention is very useful in allowing voters with weak preferences to give power over to voters with stronger preferences. As such, it can make majority rule more Utilitarian.

Abstention can to some extent be simulated, when there are only two options, by using an equal probability of voting for one option or the other. This is because if all abstaining voters do this, then with large probability their votes will cancel out.

Some countries allow abstention when choosing a Prime Minister during the Government formation process.