Could someone please explain this quote: "Who would be free themselves must strike the blow. Better even to die free than to live slaves"?

This quote is from a speech that Douglass gave in 1863.
Basically, what Douglass is saying here is that, if you want to be free, you have to
make yourself free.  You cannot wait for someone else to do the job for you.  This is
because it is better to die trying to be free than to live in
slavery.

This speech was given to try to motivate blacks to
participate in the Civil War as soldiers.  Douglass was arguing that they would not
deserve their freedom if they did not fight for it.  This idea motivated blacks to
fight, both in the Civil War and (not militarily) later
on.

This is the spirit that, for example, motivated the
people who made up the civil rights movement.  They could have waited for the white
people to decide that it was time to give blacks their
rights.  However, they did not do this.  Instead, they rose up and tried to
take the rights that they deserved.  This is just what
Douglass is advocating--if you want to be free, you have to make yourself
free.