Puppet agent 5.3.6 is a bug-fix release that includes updates for Puppet 5.3.6, Facter 3.9.6, Hiera 3.4.3 (which contains no user-facing changes), and pxp-agent 1.8.3. It also contains updates to curl, and no longer ships as a package for Fedora 25. For details, see https://puppet.com/docs/puppet/5.3/release_notes_agent.html.
PuppetDB 5.1.5 is a minor bug-fix release. For details, see https://puppet.com/docs/puppetdb/5.1/release_notes.html.