A learning curve is a hypothetical graph of how much time and energy must be invested before a tool can be used productively by the average user. If a tool has a high learning curve, it requires substantial study and experimentation before it will actually be useful. If a tool has an extremely low learning curve, the average user will be able to pick it up and intuitively know how to use it.
A lower learning curve is generally prefered, but sometimes a higher learning curve is acceptable if it results in a more powerful and flexible tool.
As an example, the Macintosh is generally considered to have a smaller learning curve than Unix.