Laissez faire is short for "laissez faire, laissez passer" a French phrase meaning to let things alone, let them pass. First used by the eighteenth century Physiocrats as an injunction against government interference with trade, it is now used as a synonym for free market economics.
The laissez-faire school of thought, or libertarianism, holds a pure capitalist view, that capitalism is best left to its own devices - that it will dispense with inefficiencies in a more deliberate and quick manner than any legislating body could. The basic idea is that less government interference makes for a better system.
Laissez faire (imperative) is distinct from laisser faire (infinitive), which refers to a careless attitude in the application of a policy, implying a lack of consideration, or thought.