The rpm normally has the binary package (the one that you actually executes)
The src.rpm normally has the source code (from which the binaries are compiled)
An 'end user' shouldn't normally have to install src.rpm.
If you really need to install from source (e.g. you have very specific an unusual configuration requirements) then you normally have to go to the location where the source was installed and look for a file with the compilation instructions. e.g README or INSTALL or something like that). Then follow the compilation instructions making sure that you really understands each of the source configuration and compilation parameters.
Actually for Fedora you will rarelly require to directly install the rpm. It's better to install with yum as it will also install the required dependencies.
In this case if you want to install net-snmp do the following:
- Open a command console (Terminal Program) and type the following (without my comments)
# you need to be root to run YUM, so change to root
# Execute yum
yum install net-snmp
and that's it!