In the political context, “abstention” often refers to abstaining from voting – that is, deciding not to vote for whatever reason. Abstaining from voting can often be a form of protest (if none of the options are seen as suitable), though it may well be the case that the voter simply doesn’t feel knowledgeable enough about the situation, or in a parliamentary context, that there would be a conflict of interest if the MP were to vote on a subject.
It’s important to note, though, that abstaining is not the same as casting a blank vote. A blank vote occurs when a voter returns the ballot paper either blank (hence the name), or intentionally spoilt. Abstention is when a vote is not cast at all.