A filibuster is a tactic historically used by members of a minority party — Democrat or Republican — to prevent bipartisan legislation from being rammed through the Senate by the majority party. A filibuster can only be stopped by a 60 vote majority, a tally that typically requires partisan support to achieve.
The really sad thing about Harry Reid’s threat is that it utterly exposes Harry Reid for what he is: a hypocrite. The proof is in Reid’s own prior opposition to limiting filibusters.