Short for Impression Load Deflection, ILD is a measure of the softness or firmness of mattress foam. If you really want to get technical, it's the amount of pressure needed to indent a 4-inch piece of foam by 25%. The higher the ILD, the firmer the foam.
While you'd be unlikely to make a mattress choice based on ILD, it can be useful as a way of comparing how different products might feel. This information should be listed in the mattress specs or on the manufacturer's website. Loom & Leaf's foams have an ILD between 14 and 44.