An extremely strong beer made through a process called "freeze distilling," which separates the water from the alcohol and sugars to concentrate them.
When the frozen water (ice, duh) is removed, the remaining beer is concentrated, thus making it stronger than it was before. Freeze distillation is possible because water has a lower freezing point than ethanol, so the water freezes and leaves behind the liquid alcohol.
The possibly apocryphal history of Eisbock is that it was accidentally created when an apprentice at Reichelbrau brewery (now part of Kulmbacher) left a barrel of bock beer on an especially cold winter evening instead of moving it to a warmer part of the brewery as instructed. The frozen water expanded and broke the staves of the barrel. When the head brewer arrived at the brewery the next morning, he found a block of ice around a small amount of concentrated bock beer.