Opacity is just that: A percentage determines how transparent that layer will look above another layer in a layer stack.
The official Adobe help for Photoshop can help explain the shorter list of Blend Modes PS Touch has including Screen and Multiply.
Match Color will try to capture the overall color tone of the layer beneath it. (Especially good for trying to match two disparate layers.) Take note this only shows up for the next to bottom-most layer and above in the layer stack for obvious reasons. (You can't match color in the bottom-most layer because there isn't anything underneath that layer to match color with.)
Merge Layers does what you think it does with some options: You can merge only visible layers, flatten the whole project into one layer or simply merge the selected layer with the layer immediately beneath it.
Finally, Delete simply deletes the layer.
Thanks for the explanation, I'll try the options with a bit more