By that they mean that our ability to perform useful work converting energy decreases each time we let energy end up as heat which is dissipiated into the universe. Useful work is called exergy by physicists. Since the vast majority of wind turbines produce electricity, we usually measure their performance in terms of the amount of electrical energy they are able to convert from the kinetic energy of the wind. The rest of us may still call them wind turbines. Mixing up the two units is a very common mistake, so you might want to read the next section on power to understand the difference.
Power is energy transfer per unit of time .
Power may be measured at any point in time, whereas energy has to be measured during a certain period, e.g. a second, an hour, or a year. If a country like Denmark has, say 1000 MW of wind power installed, that does not tell you how much energy the turbines produce. In order to find out how much energy the wind turbines produce you have to know the distribution of wind speeds for each turbine. In Denmark's case, the average wind turbines will return 2,300 hours of full load operation per year. 2.3 TWh of energy.