At first, Boomla had a few fixed page layouts, which included the menu, the page background color, etc. That was very limiting, especially when you wanted to create modern websites that often included full-width content blocks.
To address that, the Boomla Theme that was released ~2.5 months ago came with a very stripped-down page app: it contained nothing. One could finally define all the elements on the page by adding them one-by-one, including the menu and the footer.
This brought complete flexibility at the cost of a maintenance hell: if you had 50 pages, you would end up with 50 individual menu elements, each with their own configuration. As a consequence, if you wanted to change them, you would have to apply the change 50x! Obviously, that's not the way to go. This was in practice limiting Boomla to be used for very small websites.
Components allow you to reuse those elements in multiple places. Turn an element or element structure into a component and then you can create instances of it. A menu component instance would simply display the menu component in all those place where you want to reuse it.
That would be in itself extremely cool, but there is more.
Using components, you can create centrally managed page layouts. (Some systems call it page templates.)
To create a page component, simply turn a page into a component and define where page-specific contents should go. After that, you can create multiple page instances that share the same layout.
Every organism has some kind of a shell to protect its parts from the external world. We humans have a skin. So do component instances: you can't edit them directly and that's important.
If we could edit the component instances directly, we would make changes to them without realizing that we are editing the same element in lots of other places. To prevent this, you have to explicitly open the component editor first. This means that the elements that make up a component instance are not selectable, nor editable, except in the component editor.
The major consequence of this is that now you can create much larger websites with Boomla, without being lost in a maintenance hell. And critically, you can do this without writing a single line of code.