You know that Jambient plays loops. But patterns let you set it up so that the loops play differently at different times. For instance, you could set your jam up so that a sample of tapping on metal repeats three times, then falls silent for two cycles, then plays again, but this time very quietly, then once again but this time at a higher than normal pitch. Meantime, each time it plays the sample, it could move the pan of the sound from left to center to right and back again.
Patterns allow this to happen. Patterns also allow you to set several loops of differing lengths playing on the same interval, so even if loop A is 3 seconds, loop B is 3.23 seconds and loop C is 4.16 seconds, you could set them all to repeat on a 4.67 second interval. For those sounds that are shorter than the time set, Jambient inserts silence.
Patterns are one of those things that can seem confusing until you actually use them. Our suggestion is to get one or two samples playing in Jambient, and experiment with the controls demonstrated in the Flash movie here. As you experiment, it'll become clear how to use Patterns, and how powerful they can be.