Every SKP [model] has a 'default' layer - it's called 'Layer0' in English, and the equivalent names in other locales.
By default its the active [current] layer and it's visibility is 'on' [you can't switch the active layer's visibility 'off'].
So at its simplest every bit of geometry [lines/faces/etc] that you make are on made on 'Layer0'; as are any groups or components that you might also make/add to the SKP.
You can make new layers at any time and set their visibility to 'on' or 'off'. You can 'move' objects to any another layer by selecting the object[s] and then using 'Entity Info' or the 'Layer Toolbar' drop-down list. If their new layer is 'off' then the objects will 'disappear' until that layer is turned 'on' again.
The on/off settings for layers can be saved with different scene-tabs and thereby you can navigate around you model seeing just what you want - for example if all of the 'roof-level' layers are turned 'off' then you can see the things on the lower-levels more easily...
You can reset the 'active layer' to any layer... BUT this is dangerous, as anything you make subsequently will be on that layer and you might not want it to be there. Later when you switch layers on/off things could disappear unexpectedly - including partial geometry - this could still be linked to visible geometry, so if a face is on one layer and its edges are on another layer you could hide the face by switching off its layer but its edges will remain visible - now as these edges then seem 'unfaced' erasing them might seem attractive... BUT the temporarily hidden face on the 'off' layer still relies on these edges for its very existence and so it will erase with them - when you switch that face's layer back on it's not there surprise! So it is recommended that 'raw' geometry is always made on Layer0 [or moved onto it]. It just so much easier to get your head around!
Unlike CAD - where layers separate geometry and visibility etc - Sketchup's layers are used only to control their members' visibility.
Groups or components placed on different layers will be seen or be hidden as the layers' visibility settings are changed.
Exceptions to the rule... Some 'un-grouped' entities like Text or Dimensions will probably be easier to manage if they DO have their own layer[s] which you can turn on/off - I'd still recommend making them on Layer0 initially and moving them onto their own layers later, there are several 'selection' tools to help select them en mass, but if you temporarily switch the 'Text' layer 'off' and highlight Text entities and Layer toolbar them to that layer they disappear as you go on - when there's no Text left it's all been moved over. If you save layer settings with scene-tabs you can easily revert to those layers - this makes it easy to have all layers temporarily 'off' except Text [and the active layer Layer0] and then you can check that you haven't accidentally moved say a Door onto the Text layer !
Another exception is 'color by layer' - personally I never use this but with some imported CAD data it can be useful - then everything takes its material from its layer, so placing stuff onto the right layers becomes important... Don't try this unless you are supremely confident!
Another use of layers is 'pseudo-animation', where you have copies of an object, e.g. a component, each on a different layer and in a different location, by swapping their layers' visibility through a series of scenes, so that only one version of the object is seen, it will look as if it is one instance moving as you animate through the various scene-tabs in order - there are several tutorials covering this...
Hope this has helped...
