Cair Andros was a large river-island in the Anduin River, to the east of the land of Anórien. It was held in defence against Mordor by Gondor to prevent crossings of the Great River, though these defences were breached during the War of the Ring.
The island itself was long and narrow, a little over ten miles in length. At its northern end, sharp rocks split the waters of the Anduin in a bubbling foam that gave the island its name. Cair Andros was covered with trees for much of its length.
Strategically, the island was of immense importance: apart from the bridge at Osgiliath, it was the only practical means for an army from the east to cross the Anduin into Gondor's heartlands. The Gondorians took this risk seriously. Amon Dîn, the first of the Beacon-hills, was set up originally to warn the citizens of Minas Tirith if Cair Andros was ever breached.
Gondor took steps to fortify the island, too, but the history of these fortifications is difficult to establish. We know that it was manned at the time of King Ondoher's fall in Third Age 1944, but whatever defences were in place at that time were apparently abandoned, since Denethor's great-grandfather Túrin II found it necessary to fortify the island again in about the year 2900.
The Gondorian defences fell on March 10, 3019. On the same day that the Witch-king of Angmar rode from Minas Morgul with the armies that would besiege Minas Tirith, Sauron released a smaller force (though still numbering more than 6,000 Orcs and Men) from the Morannon. They overwhelmed the defenders of Cair Andros, and used the island to cross into Anórien. It was this northern force that blocked the eastward progress of the Rohirrim as they rode to Gondor's aid, though after the Battle of the Pelennor Fields, the Rohirrim chased them down and pushed them back out of Anórien.
They retreated to Cair Andros, and apparently held the island for some days. In his march on Mordor, Aragorn released a small group of soldiers from his main army to retake the island if necessary, and hold it. They must have succeeded, since the last we hear of the island of Cair Andros, its moorings were used by the ships of Gondor, as glimpsed by Frodo after completing his Quest.