The web editor is one large bootstrap column, this web component allows you to create columns within that column.
If you had a long web component, for example a Size 3 Side Menu web component, the item to the right of it would appear but the next item would appear below the Side Menu.
A column allows you to fill that space more efficiently, create a column next to the Side Menu size 9 and now all the items you add to the column will appear to the right of the side menu.
See this example that shows this representation:
In the Mition website editor, the column web component looks like this (see below image), with a [+] icon that enables you to add new items. You can also adjust the order of items up / down within the column web component.
You can also stack column components side by side in any configuration (e.g size 6 and size 6). Add a custom CSS class to the column and target the whole row for custom CSS.