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A PCB 'fence' (sometimes called 'picket fence') is a row of closely spaced vias connecting two ground planes. A signal layer is located between the two ground planes. The idea is to construct a vertical ground alongside a signal trace to isolate that signal trace from other traces or structures on the same signal layer.
A via fence is generally not necessary if you have carefully laid out your signal traces, and you have paid close attention to the signal return path. If improperly used, the via fence can create resonant loops that give rise to more signal degredation than would have existed without the fence. This is particularly true with high speed edges on digital signals because the rapid rise or fall of the signal edge represents a broad range of frequencies, any of which may find the fence as a resonant loop nearby. Fencing can also create mini-waveguides in analog circuits at microwave frequencies that actually increase coupling instead of reducing it.
You will find some board designers that use 'fences' on all critical signals because somebody told them it was a good thing to do. They don't really understand what they are doing, but it looks like they have done "something" to reduce crosstalk. The ONLY way to verify that your fence will actually help and not hurt, is to simulate the fence structure with a 3D field simulator program