The term comes from the UK parliament. Members of parliament sit on benches that face each other. The party that forms the government of the day(currently the Labour party) sit on the benches to the right of the speaker. The party in opposition(currently the Conservative Party) and the various third parties(the Liberal Democrats, etc.) sit on the benches on the speaker's left. Party leaders, either government ministers or members of the opposition shadow cabinet, sit on the benches at the front and are known as "frontbenchers." Rank and file members sit on the benches behind the party leaders and are known as "backbenchers."
A good description of what a backbencher does can be found in the book Commons Knowledge: How to be a Backbencher by Paul Flynn, MP. Mr Flynn is a Labour MP for the Welsh constituency of Newport West. I don't think you can buy the book in the US but you can order it from bookshop.parliament.co.uk.
I chose this as my blog title as nod to my political junkiness and my fascination with the UK parliament