The Columns Block allows you to insert a row with multiple columns. When you first insert the Columns Block, you’re prompted to select a layout from a selection of commonly used layouts. You can also choose to skip this step and you’ll get the default layout which is two 50% width columns. After adding a Columns Block, you're then able to add whatever blocks you want, into those columns. The Columns Block is useful for making more interesting layouts with your content, rather than the typical single column of text that the Classic Editor allowed for.
You can select the number of columns within the Columns Block using the slider in the Settings Sidebar. If you select an individual column within the Columns Block, you can change the width of that column using the Percentage width slider in the Settings Sidebar.