Coherence is a measure of how well a signal, such as a optical wavefront, correlates with itself. For example, if you measure a peak at one point in space and time, what is the chance that you will measure a peak at another space and time? This hints that there are actually two forms of coherence, one related to time and the other to space.
Temporal coherence looks at how well radiation measured at one single point correlates over time. In other words, if you measure a peak at one moment in time, how well can you predict that you'll measure a peak at another moment in time? Temporal coherence generally requires a small spread in wavelengths and a source which emits light in-phase. Lasers typically have high temporal coherence, while sunlight, which has a broad emission spectrum, has a low temporal coherence.
But that's not the end of the answer.
The other type of coherence is spatial coherence, and relates to how well two points on an emitter are correlated. One classic way of demonstrating spatial interference is the double-slit experiment: put two small slits in a sheet, and check to see that the light from the slits interferes constructively. Spatial coherence generally requires a small degree of angular spread. Again, most lasers have high spatial coherence. Sunlight also has high spatial coherence: because the sun is so far away, the rays of light are almost parallel.
The coherence of sunlight has been studied since 1869 (Agarwal et al, "Coherence properties of sunlight", Optics Letters 29, p. 459, 2004) -- but even with more than a century of coherence, the subtle difference between spatial and temporal coherence can be tricky.
