One thing that you want to keep in mind is that if you merge a layer, and you close the work or Gimp, you won't be able to get that layer back. I don't know If I explained clearly, any layer you merge or any effect you put on a layer you cannot roll it back when you close Gimp. Even if you save your work.
Yes thank you Conbagui, I understand what you are explaining, thank you all so much for all the help me understand the little ins and outs!!
You can 'swop' layers between opened images without destroying or 'loosing' layers.
You may wish to practice this by creating two images open at the same time each with 2 colored layers and by holding down a left click of the mouse on any layer, drag it over the other image to activate it and release the mouse, now your first image still has 2 layers, your second has three.
Just have a play, here's a picture description.
Remember, with Gimp there are very often more than one way to achieve things. "Where did my tool options go? grrrrr"
This one stumped me quite a few times RJKD, thank you, that is very helpful to know!!