Both mean to hold back or keep in check/ control. Sometimes people are subdued by force.

As Jura says - they do both mean keep in check or keep under control. However 'restrain' is a physical restraint. So - as Jim says, you can restrain a moving object. Subduing however, implies some mental control as well. You can subdue someone with sedative drugs. You can subdue someone (who is rational!) with a threat of violence, or by restraining them. That is why (as Jim says) you cannot subdue an inanimate object.