Sinn Féin is an Irish republican political party active in both the Republic of Ireland and Northern Ireland. Its central aim is for a united Ireland. It opposes Westminster’s jurisdiction in Northern Ireland, and its oath to the Queen, so its MPs abstain from sitting in parliament.
In the 2017 election, it won seven seats at Westminster, all of them abstentionist. The MPs work for their constituents, and every so often visit Westminster to use its facilities and meet with government ministers, but refuse to sit in an institution they do not see as legitimate.
There have been suggestions that the Sinn Féin MPs would take their seats to counterbalance a government propped up by the DUP (a unionist Northern Ireland party) MPs. But Sinn Féin denies this.
The DUP has ten MPs who sit in parliament, and could prop up a Tory minority government.