What does mean a rail to rail output opAmp?
"Rail-to-rail" is a marketing term used to describe an op amp whose dynamic range is able to reach the extremes of the supply voltage. This can refer to either the output or both the input and output.
It is not possible for the output to exceed the positive or negative supply voltage (which is why these are commonly referred to in US-English as "supply rails"). This is one of the key differences between the ideal op amp model and the real article.
As a counterexample, the ancient LM741 op amp is not "rail to rail", and here's why: in the datasheet Electrical Characteristics table, the Output Voltage Swing is rated +/-16V under the test condition of supply voltage=+/-20V. So the output can only get within about 4V of the supply rails. (There is some dependency on the output load resistance.) This is the key specification you can check on the Electrical Characteristics table pertinent to rail-to-rail output.
This limited dynamic range is very problematic at lower supply voltages. Attempting to operate the LM741 from +/-5V supplies, leaves only about +/-1V of dynamic range for the output. And operating LM741 from a single +5V supply leaves no dynamic range at all: the output is generally stuck in the middle and LM741 is not usable at such a low supply voltage.
It's also possible for some op-amps to accept inputs that are at or even beyond the rails -- Maxim Integrated makes a line of op amps marketed as "beyond the rails" (for input).
Rail to rail means that the op-amp inputs and outputs can operate near the supply voltages. Many op-amps that operate on relatively high voltage rails (I.e. +/-15v) can only drive the output to within 3 or 4 volts of the rail - for example, with a bipolar 15 volt supply, the amp may only be able to drive up to +/-12v. Same goes for inputs. The op-amp input circuitry in a rail to rail op-amp has to be designed to operate correctly near the rails. Rail to rail op-amps are especially useful when you have to use low power supply voltages. In fact, some op-amps will pretty much stop working altogether with low enough supply voltages as it won't be able to drive its output at all.
So, to sum up:
A rail to rail output op-amp is capable of driving its output very close to the power supply rails, while a rail to rail op-amp additionally supports input voltages near the power rails.