The context is Brexit and its consequences.
Perhaps this question is obvious, but in my country it is difficult to find information about this topic.
Could you please explain me in short what these people are for?
These terms refer to the two groups of people who decided the “Brexit” referendum (Should the United Kingdom begin the process to leave the European Union?).
“Leavers” voted for it, that is to leave the EU, and “Remainers” voted against, that is to remain part of the EU.