Your pages of formed of content blocks.
When you drag and drop new content blocks onto your page, any existing blocks resize to make space.
Advanced concept: When this is done, a new "container" is created containing any blocks with are resized plus the new block.
The page is split into 12 virtual columns. When blocks are automatically resized, they "snap" to the nearest columns boundary.
This feature exists to help you align various page elements.
What this means is that when you have 2, 3, 4 or 6 (each of which divides from 12 evenly) blocks on the same row, all the blocks get evenly distributed.
However, if you have 5 blocks side-by-side; 3 blocks "snap" to be 2 columns wide, and 2 blocks "snap" to be 3 columns wide.
Don't worry. You can manually resize blocks to be any width that you want.