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. Saatva's foams have an ILD between 18 and 30.