This is a new technique introduced by Loom 0.11 to add methods into a specific existing class. More specifically, you can create an Interface, and then inject this interface into the class. As result the target class will acquire all the methods of the interface, as if it always had them. Interface injection is a compile time only feature, this means that a Mixin should also be used to implement the interface into the target class.

This is particularly useful for libraries, with this you can add new methods to existing classes and use them without the need of casting or reimplementing the interface every time.

Let's explain better with an example:

The scope of this example is to add the following method into net.minecraft.fluid.FlowableFluid to get the sound of the bucket when emptied. This, normally, is not possible because net.minecraft.fluid.FlowableFluid does not have a similar method.

Optional<SoundEvent> getBucketEmptySound()

To add the method into the class, first of all you need to create an interface with it:

package net.fabricmc.example;
 
public interface BucketEmptySoundGetter {
	// The methods in an injected interface MUST be default,
	// otherwise code referencing them won't compile!
	default Optional<SoundEvent> getBucketEmptySound() {
		return Optional.empty();
	}
}

Now you need to implement this interface into net.minecraft.fluid.FlowableFluid with a mixin implementing the interface:

@Mixin(FlowableFluid.class)
public class MixinFlowableFluid implements BucketEmptySoundGetter {
	@Override
	public Optional<SoundEvent> getBucketEmptySound() {
	    //This is how to get the default sound, copied from BucketItem class.
	    return Optional.of(((FlowableFluid) (Object) this).isIn(FluidTags.LAVA) ? SoundEvents.ITEM_BUCKET_EMPTY_LAVA : SoundEvents.ITEM_BUCKET_EMPTY);
	}
}

Lastly you need to inject the interface into net.minecraft.fluid.FlowableFluid. The following snippet can be added to your fabric.mod.json file to add one or more interfaces to the net.minecraft.fluid.FlowableFluid class. Note that all class names here must use the “internal names” that use slashes instead of dots (path/to/my/Class).

{
	"custom": {
		"loom:injected_interfaces": {
			"net/minecraft/class_3609": ["net/fabricmc/example/BucketEmptySoundGetter"]
		}
	}
}

Now you can use the new method:

Optional<SoundEvent> sound = mytestfluid.getBucketEmptySound();

You could also override this method in classes extending FlowableFluid to implement custom behaviours.