While all of the above write-ups are of course correct,
filibustering can also mean something entirely different: the attempt to take over countries that were at peace with the
US by means of privately financed
military expeditions.
In the decade before the American Civil War, pro-slavery Southerners actively tried to add slave states to the Union. The first big attempt was started by Narcisco Lopez who led three unsuccessful invasions against Cuba, convinced that a popular uprising against the (then Spanish) government would follow. When this failed to happen, Lopez was executed by the Spanish authorities.
The most interesting and notorious filibusterer was William Walker, who led a number of armed invasions into Central America, starting with Baja California, then Nicaragua (several times) and finally Honduras. He was actually president of Nicaragua for a short while and even recognized by US President Franklin Pierce. He was executed by Honduran authorities in 1860.
When the American Civil War started, filibustering came to an end. Afterwards, the change from an agragrian to an industrial nation meant less need for land, so no more invasions. Nowadays, the term is only used to describe the famous delaying tactic of monopolizing the parliament floor to stop a vote from being taken.
With thanks to the Encyclopedia Brittanica for some of the data.