Wreckage is as the name suggests wreckage that is left over from battles between ships. These wreckages always contain resources but 95% of the time it wont be a lot but there is a general rule that links to the amount of resource a wreckage contains. The larger the battle (more ships and bigger GRT involved) then the more resources there will be left over. Also the more of one type of resources used to build the ships the more of that particular resource will be left over.
Wreckage can be mined but unlike asteroids they shouldnt be used as a main source of resources. They should only be used as a spare if at all. Some players dont even bother mining wreckages as they consider it a waste of hydrogen fuel and very little returns. What however some players do is equip a Salvage laser to their asteroid mining ships and whenever they come across a wreckage they mine it for the few hundred resources it may contain.