Sheet Mulching is...
...a layered mulch system. It is a simple and underutilized technique for optimizing the benefits of mulch and Nurturing the soil.

Sheet mulching can be used either in establishing landscape, or to enrich existing plantings. In both cases, mulch is applied to bare soil or on top of cut or flattened weeds.

Trees, shrubs, herbaceous perennials and annuals are planted through the mulch, or a small area is left open to accommodate established plants.

Sheet mulch can:

Suppress weed growth
Reduce labor and maintenance costs: weeds are composted in place
Improve nutrient and water retention in the soil
Encourage favorable soil microbial activity and worms
Enhance soil structure
Improve plant vigor and health, often leading to improved resistance to pests and diseases (less info)