Any time you have a question about a command on a Red Hat distro such as Fedora, CentOS, or RHEL it's best to utilize the package manager tools
If the package isn't installed then use
yum to see what it's about.
$ yum info <package name>
If it's already installed then you can also use
$ rpm -qi <package name>
$ yum info fedmsg
Loaded plugins: auto-update-debuginfo, changelog, langpacks, refresh-packagekit
Name : fedmsg
Arch : noarch
Version : 0.7.7
Release : 1.fc19
Size : 465 k
Repo : updates/19/x86_64
Summary : Tools for Fedora Infrastructure real-time messaging
URL : http://github.com/ralphbean/fedmsg
License : LGPLv2+
Description : Python API used around Fedora Infrastructure to send and receive messages with
: zeromq. Includes some CLI tools.
If you have the name of an executable and aren't sure what package it's a part of you can use the command
$ repoquery -qf </path/to/file>
$ repoquery -qf */fedmsg
So what does it do?
The link that was posted in the comments to the project's website says it best:
fedmsg (Fedora-Messaging) is a python package and API used around Fedora Infrastructure to send and receive messages to and from applications. See Overview for a thorough introduction.
So what does it really do?
If the technical jargon is too much to stomach then the bottom line is this.
fedmsg is a project to provide a messaging bus that the various tools in the packaging toolchain can use to relay information to each other as to their states for various build/packaging related tasks.
Take Koji for example. Koji is Fedora's automated build system for compiling RPMs. Here's a screenshot:
So as tasks are scheduled and built via Koji the state of where they are in the process is not easily exposed. The idea of fedmsg is to provide a messaging bus so that services like Koji can share the state of these activities with other systems within the packaging toolchain. Right now a maintainer of a package has to sit there and periodically check the web UI to see if a build has completed successfully.
The overview page has a more detailed description of fedmsg if you want more info.