Therm-a-Rest is a trademarked name for a line of self-inflating camping mattresses. They were invented in the early 1970s by two former Boeing engineers, who were avid backpackers. They named their company Cascade Designs.
A Therm-a-Rest air mattress consists of an airtight nylon fabric envelope filled with a sheet of low density, open-cell polyurethane foam. The urethane foam does not provide significant padding; when a valve is opened, it tends to expand to its normal size, and as it does so it draws air into the mattress. When the mattress has fully inflated, the user simply closes the valve. To increase firmness, a few lungfuls of air can be blown into the mattress. To pack the mattress, the valve is opened and it is rolled up tightly to force the air out, whereupon the valve is closed to prevent re-inflation. There is a slight weight penalty in such mattresses compared to non-inflatable pads, but when used properly they can provide isolation from cold and hard surfaces.
Similar mattresses are now available from competitors.