The Frezarite Crystal is obtained from a Frezarite Meteor crash site. It is used to both craft the Freezer and fuel it. One Frezarite Crystal will provide enough fuel to craft 8 items in the Freezer (similar to coal in a vanilla furnace). Note that it is not the only kind of fuel available for the Freezer though.
Fuel for the Freezer (8 items crafted)