Alderaan was a planet near the galactic core in the Star Wars galaxy. It had a representative in the Galactic Senate: Bail Organa at the time of the Clone Wars. Grand Moff Tarkin used the Death Star to destroy Alderaan as a demonstration of the station's superlaser and as the first step in implementing the Tarkin Doctrine. This was also an attempt to bully the location of the Rebel Base out of their captive, who called the planet home (though it is very clear that Tarkin was going to destroy the planet anyway).
According to Leia Organa, "Alderaan is peaceful! We have no weapons!" She made this claim when hoping to convince Tarkin not to destroy the planet. Of course, even if true, this claim does not preclude Alderaan having a planetary shield to defend it against attack.