This is the package signature as generated by The GnuPG. This signature identifies the party that created the package. By verifying it you are sure that the package coming from a trustworty person or organization.
Here is the package signature:
-----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (GNU/Linux) iEYEABECAAYFAlDYf2AACgkQqesEzEdeXBEr4gCbBhVh4eT6J0WYrDWHoAkYZmM+ FXUAoKdsEg6q+kcLUZn83rSMI92/cel+ =6o7a -----END PGP SIGNATURE-----
You can save this signature in a text file (e.g. fftw-3.3.3-i486-1gds.txz.asc) in the same directory where the package file (fftw-3.3.3-i486-1gds.txz) resides and you can verify it by issuing the following command:
gpg --verify fftw-3.3.3-i486-1gds.txz.asc
NOTE: You need to import the public key of SlackPack first:
gpg --import KEY