Tabs have become popular in modern web browsers. Instead of opening new browser windows several web pages can be opened in the same browser window. The window gets adorned with a tab for each open web page, for easy return to the specific web page. This way you get a chance to an organized browsing experience.
Layers are the K-Meleon tool for opening multiple web pages in a single browser window. Only one page is visible in the window at a time; the other pages are on layers that are conceptually either "under" or "over" the visible page. To change which page is visible, you can either click on the desired layer in the Layers Menu, or use the Layers Toolbar if it is activated. Since the layers are implemented as a plugin, you can enable or disable the plugin, as described in the configuration section about plugins, depending on whether you feel you need this feature or not.
A more technical and detailed description, also covering how to configure the layers as you wish, can be found in the advanced section on Layers.