Interplanting, also known as intercropping, helps make the most of limited garden space. There are several different ways to do this, for instance by mixing slow-growing crops with speedier ones, relay cropping (which is like succession planting, but with a little overlap between successive crops), or by growing plants that have different growth habits closely together (for instance the 3 Sisters technique).
The default spacings used in the Garden Planner differentiate between plants grown singly or in blocks (where plants are spaced equidistantly at relatively wide spacings), and plants grown in rows. The row spacing is usually used when plants are grown at quite close spacing within the row, with a wider 'row gap' between the rows, but you can customize the spacings to use a closer row gap.
If you add plants in rows rather than blocks you can achieve a 'staggered' effect that allows for more intensive growing. To use the row spacing, you need to add plants in individual rows rather than dragging out a block (where plants are spaced equidistantly).
You can do this with plants of the same type, to grow more of the same crop in the space, or with crops of different types for interplanting. Normally it's best to avoid overlapping the colored backgrounds on rows of plants, but when intercropping it is possible.