Threshold layer becomes empty

Aja,

You say you are a novice and may be getting a little confused over how adjustment layers work.

Your screenshot shows you have two layers. A layer containing pixels which in your screenshot is labelled background and an adjustment layer, in your case a threshold adjustment layer. The first contains the image pixels, the second contains only the adjustment information telling Photoshop how to display the pixels below it. The adjustment layer does not contain any actual pixels, and importantly no permanent changes are made to the pixel layer below it.

If you save your document in a format that supports layers (e.g. PSD or TIFF) then both those separate layers are retained and the document can be reopened and the adjustment layer re-adjusted.

If you merge layers or save a copy in a format that does not support layers, then the adjustments are made permanently to the pixel layer before discarding the adjustment  layer and you are left with a single pixel layer which has those adjustments permanently baked into it.

When using Photoshop, adjustments should always be checked at 100% zoom, which is not a physical size but means means 1 image pixel mapped onto 1 screen pixel. This will always match before and after merging.

Dave

In addition to what c_p asked, what format are you exporting to? And what are you trying to achieve with the a threshold adjustment layer?

~ Jane

»merging the layers makes no difference.«

Could you please post a screenshot taken at View > 100% with the pertinent Panels (Layers, Channels, Options Bar, …) visible? 

Edit: One of before and one after merging.