md5sum collision question
They mean that you send a message template to anybody you want to fool, say, a certificate request to a certificate authority. You make sure that the MD5 is calculated over N 64 byte blocks of data generated by the other party, and then the 128 byte block you pre-calculated.

Now you receive back the data generated by the other party, ending with your 128 byte block, aligned on 64 bytes. In our example that would be a certificate. Now you can simply replace all the data generated by the other party, recalculate your 128 byte block and get the same MD5.

The other party, for instance the certificate authority, could use the generated MD5 to generate an MD5 with RSA signature. As the signature only depends on the private key of the other party and the MD5, the signature would be valid for both the original and the "fake" certificate.