Washing soda aka sodium carbonate "softens" water helping other cleaning ingredients lift soil from the fabrics and suspend the soil in the wash water. The washing soda binds to the minerals which make water hard and allows the detergent to be absorbed into fibers properly to clean clothes. It's like a strong cleaning version of baking soda.

I make a strong deodorizing and degreasing laundry soap using 2 parts soap flakes, 1 part washing soda and 1 part borax.

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What Is Washing Soda?
Washing soda is a chemical compound that can be used to remove stubborn stains from laundry and is an essential component in most homemade laundry detergent for powder, liquid, or single pod formulas. It is often used in commercial detergent mixtures as well to treat hard water.

The chemical formula for washing soda is Na2CO3 or sodium carbonate. It is a salt of carbonic acid, a chemical which produces a wide range of salts collectively known as carbonates. One common source of washing soda is the ashes of burned plants; for this reason, it is sometimes called soda ash.

Washing soda should not be confused with baking soda, although the two compounds are closely related.

What Are the Uses of Washing Soda?
In the laundry room, the high alkalinity of washing soda allows it to act as a solvent in removing a wide range of stains. For home laundry, washing soda can be dissolved in water to create a presoaking solution for tough stains like grease, blood, tea, and coffee stains.
Outside the laundry room, sodium carbonate is also used by some textile artists to help dyes adhere to fabric. This is particularly useful when creating natural dyes from plants.
Washing soda can be used to remove greasy build-up from pots and pans, oven racks, and drip pans. It is very good for removing coffee and tea stains from ceramics and plastic storage containers. It can even be used in bathrooms to remove soap scum and around the house for general cleaning of hard surfaces. For cleaning, mix a solution of 1/2 cup of washing soda to one gallon of warm water.
To unclog drains, pour one cup of washing soda into the clogged drain followed by two or three cups of boiling water. Allow the washing soda to work for 30 minutes and then flush the drain well with plain water. Repeat as needed.
In the garden, washing soda works well to clean outdoor furniture and decks.
It can also be mixed (1/2 cup to two gallons of water) to create a spray to help control aphids, whiteflies, and black spot on roses.
Since sodium carbonate can be dangerous in large quantities, make sure to keep washing soda out of the reach of children and pets. You should wear gloves when cleaning with washing soda because it can cause skin irritation. It can be harmful to the eyes, cause irritation to the lungs if inhaled, and may cause abdominal pain or vomiting if large doses are swallowed.

Photo thanks to Aqua Mechanical on flickr