When you apply the brake, the pads inside the drum come in contact with the brake dust, which causes the squealing. Another cause is the absence of contact between the drum and shoe.
In addition, if the drum brake shoe is not properly lubricated, this will cause it to squeal.
Here are a few helpful hints that will help you lubricate your drum brakes properly;
Don't ever put lubricant on the inside of the drum where the shoes/pads contact the drum. This will cause the brakes to stop working, and could be very dangerous.
Use lubricant on the backing plate. You'll find small ridges where the shoe rests; these can be sanded lightly with a grinding wheel, and then lubricated.
The adjustable star-wheel that separates the shoes can be prone to freezing, so it should be lubricated.
The parking brake usually uses the rear drum brakes in your car, so lubricate the parking brake cable and any linkages or moving parts.
Make sure you lubricate the separator located about halfway up the shoe.
Use lubricant anywhere you find metal-to-metal contact between moving parts, such as where the shoes slide.
Don't use dry film lubricant where the lube might contact rubber or plastic seals.
Don't overlubricate -- you don't want excess lubricant dripping into places it shouldn't go.