PDF has a concept of layers that you can enable and disable (at least in a few viewers). This has nothing to do with ConTeXt’s [[Layers]].
There’s no other documentation yet, maybe have a look into the source sources {{src|attr-lay.mkiv}}, {{src|attr-lay.lua}} and {{src|lpdf-ren.lua}}. It’s also used in some presentation styles.
<texcode>
\input knuth
\viewerlayer[Plane]{Some more hidden text...}
\stoptext
</texcode>
You can switch the visibility of layers with the JavaScript functions HideLayer and VideLayer.
Viewer layers are known to work within {{cmd|setlayer}} (see [[Layers]]), e.g. {{src|meta-ini.mkiv}}.