This is a thin gasket used when installing catalytic combustors. If it is used in your stove, then it should always be replaced whenever a catalytic combustor is removed for inspection or replacement. Consult your wood stove manual for correct usage.
This is a universal gasket used by many manufacturers to help seat the combustor into place. It will swell when subjected to the high temperatures in the combustor chamber.
Note that some wood stove manufacturers may require the use of a special gasket, and this will be supplied with the replacement combustor when needed.